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I am using this Beowolf homebrew monster as a basis to create a Large Mountain Wolf.

It currently has a CR of 3, if I add the Pack Tactics trait (as listed below) to this monster, how does that affect the CR?

Pack Tactics

You have advantage on an attack roll against a creature if at least one of your allies is within 5 feet of the creature and the ally isn’t incapacitated.

It's hard to say

In the DMG on page 280-281, there is a table of many common features of monsters, and how these features could alter the Challenge Rating (CR).

Specifically, on page 281 it states:

Name : Pack Tactics

Effect on Challenge Rating: Increase the monster's effective attack bonus by 1.

This monster currently has a defensive CR of 1 (see DMG p. 274 for rules on calculating this) and an offensive CR which is difficult to calculate since no save DC is given for its most dangerous ability. However, since this ability is only based on a Wisdom Save, we can safely focus instead on whether an attack bonus 1 higher than its current +7 would result in an offensive CR of 6 or higher (since 5 or lower would result in an average CR of at most (1+5)/2 = 3, it's current CR).

If this creature's attack bonus had been +8, this would only be relevant if it used its "bite" action to damage. Since the bite does 17 damage per round, its offensive CR would only be 4 (See instructions in DMG p. 274). Thus, its CR would likely not increase due to Pack Tactics. However, this conclusion is complicated by the way the monster's stat block is currently written.

The monster's stat block makes this unclear

The monster's current stack block appears unfinished. There is one ability written as:

Action Name. Enter the description for your action.

And a previously mentioned (powerful) area of effect ability that calls for a Wisdom Save, but doesn't give a DC. This makes it somewhat difficult to know what calculations the designer used to determine the creature's current CR.

Critically, the unnamed action appears in the stat block at the very start of the Actions section. This indicates to me that the creature's creator may have intended to give this monster multiattack, as that is where multiattack is usually listed in the stat block of creatures that have it.

This is a serious problem, because if this creature can bite twice in a turn, then its current offensive CR would be 5 (which would fit with its current CR of 3), and a further +1 bonus to its attack would then pump its offensive CR to 6, which could increase its overall CR to 4 (DMG p. 275 indicates you round up the average of Offensive and Defensive CR).

This is further complicated by the fact that this creature's most powerful action is an area of effect power that, as written, could also damage nearby allies. Since Pack Tactics only works when allies are nearby, that means the Beowulf's Pack Tactics will present a tradeoff, gaining advantage while losing (or complicating) access to its best ability.

So which is it? Higher, or the same?

Until this creature's stat block is completed, it will be impossible to accurately calculate its CR, either with or without new features. But if the creature has multiattack and can bite at least twice, then Pack Tactics would raise it from a CR 3 to a CR 4.

The Dungeon Master's Guide discusses this

In the Creating a Monster section of the DMG (starting on p. 273), there are a number of monster features with proper adjustments listed. This includes Pack Tactics (listed on p. 281):

Increase the monster’s effective attack bonus by 1.

Then you just recalculate the Offensive CR and see if it causes an increase to the overall CR.

If your monster’s attack bonus is at least two points higher or lower than that number [suggested on the table], adjust the [offensive] challenge rating suggested by its damage output up or down by 1 for every 2 points of difference.