I've been watching one of our players repeatedly cast toll the dead (Xanathar's Guide to Everything, p. 169), across seven sessions, and a dozen different combat encounters, and the DM has never once allowed her to do any damage with the cantrip. She has a spell save DC of 16, yet our DM always "mysteriously" rolls the saving throw.

Obviously, "just quit the game", "that group is not for you", are the answers most folks will immediately suggest, but I'm not the one playing a warlock and I feel like telling her to quit would be awfully rude of me. She's a really quiet and shy person, and I can't help feeling like someone needs to stand up and defend her. Last session she looked like she was on the verge of tears.

Anyone have a creative method of calling your DM out for being a dice cheat in front of the entire group? I'm really disgusted by his behavior and I'm guessing that statistically speaking the permutation is so large by this point that his monsters have won the powerball ten times over.

Don't "call them out", it's about Communication

As with most interpersonal issues, the focus should be on communication. You can absolutely talk with your DM about it, but you should talk to the other player first.

The concerned player

Right now, you haven't talked directly about this concern with the affected party. Before you do anything, you should do so. Acting on someone else's behalf without informing them of doing so may cause more problems then you think you're solving.

Discuss with them how they feel, if they feel there's a problem, and what, if anything, you could do to help.

The DM

If the player wants you to talk to the DM, wants to talk to the DM together with you, or wants to talk to the DM alone, let them take the lead.

You can mention what you perceive at the table and that there is concern that it's taking away the fun for someone. Is there something in-game happening that you don't know about because it's plot related (and they can't tell you?) Give the DM a chance to talk without accusing them of anything.

Bringing up open rolling is a solution, but you haven't yet had the DM acknowledge there is a problem. Give them a chance to talk and you might be surprised.

If it does turn out to be negative, then you, the concerned player, and the rest of the table can discuss how you'd like to proceed.

One, don’t “stand up for” anybody without talking to them first

It’s possible that this is someone who doesn’t know how to stand up for herself, and will appreciate the assistance. Far more likely, however, it will just humiliate and anger her—“standing up for her” without talking to her first is condescending (implies she cannot manage for herself) and incredibly disrespectful (stealing her of her agency and right to decide how to manage her own affairs). That is how a child is treated, and then usually only by a parent. Don’t treat a friend that way.

So ask her if this has been bothering her—you say she seemed to be on the verge of tears, so you certainly think so, but don’t assume. You could have misread the situation, maybe (I don’t know you or how good a judge of “on the verge of tears” you are), but even if you didn’t, for all you know she could be upset about something else, and anyway she may not appreciate knowing that her emotions were “on her sleeve,” so to speak. Getting upset and emotional about things can be embarrassing—it sounds like an upsetting situation, so she should have nothing to be embarrassed about, but nevertheless most people would be.

Ultimately, this is up to her. If she doesn’t want a confrontation—and since this is about her character, she would be pulled into any confrontation you started—you need to respect that. There are alternatives here; for example, eldritch blast is generally a much-superior warlock spell over toll the dead, and since she rolls the attack roll you can all see it. The DM can still bluff about the target’s AC, but since presumably she is far from the only person making attack rolls, he would have to be consistent about that. If she rolls a 24 and misses, and the next person rolls an 18 and hits, that’s pretty blatant. (And if the DM does get that blatant, she may become more comfortable with confrontation.)

In short, if you want to support her, you must support her, which means abiding by any decisions she makes here.

Second, “calling him out” is fairly unlikely to get useful results

That sounds likely to just make someone defensive; if he doesn’t deny it and defend himself, he may feel as though he is admitting to cheating—even if he is, he very likely doesn’t want anyone to know it.

And more to the point, you don’t know that he is cheating. The math, as Xirema’s excellent answer discusses, certainly suggests that something is wrong, but “cheating” is only one possible answer out of many. If nothing else, Hanlon’s razor easily applies here: it may very well be that he doesn’t realize that he’s making some serious mathematical error here. Treating a 3.5e creature’s Will saving throw as its Wisdom saving throw would certainly do it, for example.

If you ignore this, and go straight for accusations, your best case scenario is that the group decides you’re right, the DM is lying, and they stop playing with him. If someone else from the group is willing to DM, then you can maybe have a game without him. But that’s a lot of if, and it would be massively disruptive—and that is the best case scenario. The worst case scenario is something like humiliating the person you were trying to help, alienating your friends, and becoming uninvited from the game yourself.

Third, there may not be good outcomes available here

You are talking about someone who, you suspect, cheated and as a result (the intended result?) pushed one of his players “to the verge of tears.” That is a rather extreme abuse of the DM position. If it was merely a mistake—hope it was!—and he’s willing to cop to that and fix things—hope he is!—then all may be well. If not, though, you have a severe interpersonal problem here. If you are correct in your suspicions, you may very well want to strongly consider you stopping your play with him. His behavior towards your fellow player has clearly bothered you. And even beyond that, if he is willing to treat one player like this, how long until you draw that ire? Not long, is my guess, once you start questioning it.

And that might be the best way for you to support her—if you offer her the solidarity of also being willing to leave the game over this, so she isn’t alone in being bothered by it, that may make her feel better, may improve her willingness to leave herself. It may impress upon your fellow players, upon the DM, that this isn’t acceptable. It may be that the entire group leaves—that is, that the DM is effectively booted by having his players no longer willing to play under him.

Again, this is a pretty dire result, and may again be a best case scenario. Care is required here. Your fellow player has reason to be reluctant to get into this confrontation.

Fourth, I apologize but it must be said—don’t be a creep

First, please don’t take this personally. This isn’t even directed at you, per se—answers here are written for all who have an interest in the question and read them. So this is to anyone who finds themselves in a similar situation: be very, very careful here. Your user icon is a male face; your question uses female pronouns for your fellow player, so I presume you are male and your fellow player is female. Ultimately, that shouldn’t matter, and it doesn’t to an extent—you could be gay, or a woman who just chose to use Dr. Horrible for an icon, or she could be male despite your pronouns (or another reader could be in a similar situation except the one being bullied is male), and all of the below would still be a concern. It’s just, unfortunately, a concern with a greater likelihood in the case of you being a heterosexual male and she being female.

There is a long, ugly history of men who “stand up for” a woman—particularly without her expressing any desire for that to happen—and then thinking that this action has changed something about their relationship. It doesn’t: you are, as you were, friends. Anything you are doing here, you are doing because she is a friend and because your DM’s behavior bothered you, too. Any expectation you might have of her as a result—even simple gratitude—is inappropriate. She may not be grateful—she may very well ask you to mind your own business, or be mortified and furious if you don’t give her the chance to ask that. Even if she accepts your assistance, it must be offered in good faith and with zero expectation of reward.

And the odds are, unfortunately, rather good that these concerns are going to jump to the forefront of her mind when you bring this up. This behavior is extremely common. Women are, disgustingly, subjected to it with distressing frequency. They often have entirely too much practice in being wary about this kind of behavior.

You can’t change that, at least not immediately, by yourself, for this woman. You can’t change her past or experiences, if she like many has them. (Absolutely do what you can to push for societal change that makes this behavior less common!) So you have to go into this knowing that she may very well be wary that you have an ulterior motive. You have to respect that—you have to know why she has those suspicions, understand that it’s really not anything personal, just bitter experience. If you cannot accept her suspicions, and rise above begrudging her them, this is probably a bad idea to get involved in. You not only have to be respectful enough to genuinely want to help her just because seeing her treated in this fashion upsets you and you want it to stop, and you also have to be respectful enough to understand that she may not entirely trust that you are being as respectful as you are. If you lack the maturity or discipline to do that, you should very strongly consider dramatically scaling back your plans. Maybe it would be better to just offer some sympathy for a frustrating game and suggest hope that things will improve.

Vouch for open rolling, if you're certain he's cheating

Damn it, bad luck. How much did the monster roll?

Most DMs will roll behind their shield, which is fine and all, but you can bring up this odd coincidence in a friendly manner and ask to see the next rolls. Some DMs may take that personally, which is hardly what you want, so be sure to approach it in a respectful manner, as a player who's curious about the effectiveness of this particular spell and wants to build some strategies that depend on its effectiveness and not as an accuser.

But as Szega says on his comment, it may be you're fighting several monsters that are simply resistant to the cantrip, so it's hardly his fault.

If you're going to confront your DM about this (and make sure you discuss this with the affected player first!), you're going to want to have a case to make for why you think something's wrong with these rolls. It's all fine to say "well, they missed 10 times in a row, you must have cheated!", but D&D is a game driven by long odds and strange probabilities. You're going to want a specific case for why Toll the Dead failing as often as you claim it is is evidence of systematic rigging, whether intentional or unintentional.

The Basics of the Math

A DC of 16 is pretty close to the expected DC for the saving throws of a level 9-12 spellcaster. So it's probably fair to assess that the creatures you're facing against are appropriate for that level range (i.e. probably not tougher than a CR12 creature).

In the officially released Sourcebooks for 5e (not including Adventure Modules) there are ~630 creatures at or below CR12 (~450 if you exclude creatures that are less than CR1). Of these creatures, ~130 of them have noteworthy Wisdom Saving Throw (the type of Saving Throw that Toll the Dead uses) scores.

So in general, you can expect, through the course of your campaign up to this point, to expect that most creatures you face aren't going to have spectacular Wisdom Saving Throws. Even for creatures with especially good Wisdom scores, you would typically expect their Wisdom Saving Throw modifiers to be in the 5-9 range. But maybe your campaign is favoring said creatures.

So against a DC16 Wisdom Saving Throw, the best your enemies (assuming pretty decent Wisdom Scores) ought to be able to do is save against Toll the Dead about 50-70% of the time, meaning your teammate's spell can only be expected to succeed about 30-50% of the time.

So depending on what you're fighting, you certainly cannot expect to land that hit every single time. Saving Throw based spells tend to get resisted somewhat more frequently than Attack Roll based spells, due to Saving Throws favoring the defender (because they'll win ties) and also just having lower thresholds for success (for the defender) than Attack Rolls usually do.

The Math suggests some odds are being altered

At 50% odds to hit, the odds of missing 4 times in a row is 6.25%, or about 1/16. 8 times in a row is 0.39%, or 1/256. At 12 combat sessions, if we assume they got to use the spell twice per combat session (which is a bit on the conservative end), the odds of missing with this spell 24 times in a row is about 0.00000596%, or 1/16,777,216.

The odds are higher at 30% to hit, where you'll miss 24 times in a row about 0.019% of the time, or 1/5,219 odds. At 40%, you'll miss 24 times in a row around 0.00047% of the time, or 1/211,042 times.

These are, admittedly, a pretty extreme range. 1/5,219 odds are low; but given how many people play D&D, it's possible you/your group were just the unlucky ones that fell victim to the tail end of the probability curve. It happens. A one-in-a-million chance isn't literally impossible to happen: it happens once every million trials (on average), and someone has to be that lucky (or in this case, unlucky) millionth person.

But these are pretty long odds. Literally, >99% of the time, your DM is screwing you over.

What to pay attention to

So if your claims about the outcomes of these dice rolls are accurate, you need to work out whether your DM is literally lying about the results of dice, or giving his monsters unfair stats. Here's a useful table:

beginarrayl
hline
Odds of Hitting & Odds of Missing & WIS Saving Throw & Odds of Missing 8 Trials & Odds of Missing 24 Trials \ hline
60% & 40% & +3 & 1/1525.88 & 1/3552713678.8 \ hline
55% & 45% & +4 & 1/594.7 & 1/210329248.15 \ hline
50% & 50% & +5 & 1/256 & 1/16777216 \ hline
45% & 55% & +6 & 1/119.43 & 1/1703316.89 \ hline
40% & 60% & +7 & 1/59.54 & 1/211042.53 \ hline
35% & 65% & +8 & 1/31.38 & 1/30908.61 \ hline
30% & 70% & +9 & 1/17.35 & 1/5219.72 \ hline
25% & 75% & +10 & 1/9.99 & 1/996.62 \ hline
20% & 80% & +11 & 1/5.96 & 1/211.76 \ hline
15% & 85% & +12 & 1/3.67 & 1/49.42 \ hline
10% & 90% & +13 & 1/2.32 & 1/12.54 \ hline
5% & 95% & +14 & 1/1.51 & 1/3.42 \ hline
endarray

Around the point where the odds hit 20% chance to hit, the math starts to favor the "plausible deniability" of your DM. A 1/200 chance of missing all 24 rolls is still quite improbable, but if it happened, you wouldn't accuse the DM of altering rolls; the chance of rolling a 1 when rolling advantage is lower than that. At 5%, it happens about 1/3.42 times, or around 29%: that's pretty likely!

Granted, this doesn't absolve the DM of "cheating" by any stretch; it just means that their Die rolls probably aren't the culprit. Instead, what might be happening is that the DM is making the Saving Throw modifiers for his creatures far too high. And it doesn't take much in 5e: a Saving Throw modifier of +11 is all it takes to make the 20% chance to hit realistic, and at +15, it becomes literally impossible for a DC16 saving throw to be failed (since 5e doesn't have Critical Successes/Failures for Saving Throws).

Now, in 5e D&D, a +15 to a saving throw is pretty absurd for a non-legendary creature. That's even too high for most Ancient CR20+ dragons, whose Wisdom Saving Throws cap out around 9-12. The highest Wisdom Saving Throw I could find in the 5e sourcebooks was Zariel, a CR26 Devil with a modifier of +16.

So if your DM is "rolling fairly", it's with creatures that have wisdom scores as high as legendary Fiends. Which begs a (possibly overlooked) question...

Did your DM come from 3.5e D&D?

I'm framing this like a question, but it ought to be clear it's rhetorical: whether they did or did not, these numbers would be a lot more plausible if they were using the kind of power curve expected by that game, rather than what 5e expects. A Wisdom Saving Throw modifier of +15 would still be pretty high for that game, but it would no longer be in the realm of literal gods.

What to do

So if you're worried about your DM being confrontational about their statgen, you can use this as a way to get them to open up: "Listen DM, based on how many times that player's Toll the Dead spell has failed, I think you may have set these creature's Wisdom Scores too high. Did you use 3.5e stats when generating these creatures?"

If they deny it, that makes a pretty strong case that they've been cheating with their dice rolls. If they confirm it, then you have a solid case to take to them: "Alright, I understand why you did that, but DM, you've given them the stats of a Legendary Demon Lord! Surely you agree that's a bit unreasonable for characters of our level?" Remind them of Bounded Accuracy and how that concept interacts with the 5th edition power curve, and encourage them to make sure the creatures they generate are stated appropriately for the characters they're facing off against.

However they react beyond that is probably a topic to take to another site, like SE.Interpersonal, which is specifically for resolving social disputes—which, at that point, would be what you're dealing with.

Play it totally straight, within the game's own setting. If a spell designed to hurt creatures is well and truly never hurting its targets, then you've got a reasonable claim that by now, the PCs should have noticed. "Hey, you know, it's really weird that this spell isn't working right. Is something interfering with our magic? We should investigate to find out why". And start pushing toward a player-driven sidequest to find out what this mysterious interference is.

It is possible that the DM may actually want you to do this. Sometimes these cases where things don't work the way you'd expect them to are part of the plot. Even in this case, the DM really should have dropped some more overt/less mechanical plot hooks by now, but sometimes DMs forget to do that.

To give an example of a DM forgetting about this, I once ran a 3.5e Ravenloft adventure with an NPC traveling with the party. There's a rule in that module that ordinarily, the BBEG and his minions never attack the NPC. My players eventually caught onto the fact that the NPC was never being attacked, but got frustrated because they didn't catch on to the plot reason for it. I hadn't dropped enough story hooks to make that clear. This happened right before a mid-boss fight where the BBEG was present, so I had the minion get frustrated and attack the NPC once anyway, and then the BBEG got mad and punished the minion. Once the players understood that the BBEG had forbidden his minions from attacking the NPC they were much more okay with it, and even found ways to use those standing orders to their advantage.

Embrace Failure

Our DM is not fudging rolls, but in our group we have one character who almost never hits with his long sword, and my character has some save spells which have never succeeded. This has led to some very interesting character development. Our Paladin now primarily throws a dagger and is taking feat to improve his dagger throwing, and our Sorcerer questions his power and self worth which makes him more hesitant and has changed the way I choose new spells.

Advice from my experience is that she should use those misses to add depth and development to her character. Wonderful and unexpected things can happen when you embrace failed rolls and use them to enhance your RP and character development. You could ask how Toll of the Dead failing so often is affecting her character mentally to get her thinking about it.