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An adventurer has come into possession of an invisible sword. How advantageous in a duel or skirmish would this be? Good for a one-time trick, a significant advantage throughout a fight, or even a disadvantage?

Some background detail:

Setting: Low-magic, late-medieval setting. Other strange artifacts like this might exist, but no spells or healing potions, etc.

Adventurer: Experienced adventurer and duelist, but average strength and size, and no sword-master. Personality is more suited to tricks, gimmicks, using wit etc.

Fights: Duels and skirmishes. No large-scale battles but conflict more on the line of a duel for honor, a bandit raid on a small party, etc.

Options: If there are any specific things that would make it more useful, such as type of sword, having the handle visible, any extra equipment, feel free to mention.

Very Advantageous. But with careful use.

Obvious use is assassinations, since it can let your guy appear to be unarmed. He can even do it in public, and frame a nearby armed person.

For a duel, he will need a visible weapon to maintain pretenses. Then he can sneakily stab the opponent who is focused on parrying the visible weapon.

For extra equipment, can try attaching a visible dagger to it, to explain away the swinging and stabbing motions that the wielder is doing.

Disadvantages

• It cannot be used to threaten somebody, or deter them from attacking.




• The wielder himself cannot see it, which can get dangerous, especially if he is trying to parry with it.




• He might get accused of using magic, which will have consequences.




• If the sword gets bloody, will blood be visible? If so, he can only use it once per fight.

A very good fighter is able to gauge their opponent's range and uses that to both predict/defend against attacks as well as strategizing their own attacks. An invisible sword would be much harder to gauge and thus create a significant advantage for the wielder. This was used in the anime series "Fate/stay Night" where King Arthur's noble phantasm (special power) was being able to shield the length of Excalibur.

Edit:
The primary advantage would be in 1 on 1 duel type settings. This would have little advantage in a large group skirmish as one rarely has time to stop and closely evaluate range in that kind of setting. Additionally, it would actually be a determent in an intimidation type scenario. With intimidation, size matters. Since you can't show just how impressive your weapon is, you can't use it to intimidate.

Bald Bear basically nailed everything I was going to answer except for one particular other use I saw once in an anime called Get Backers.

In the episode I watched, a bad guy got into a swordfight with one of the main characters. The hero kept being hurt by the sword even though he as pretty confident in his skill to dodge the blows. Still, he kept being hit time after time.

The explanation for that is that the attacker's sword was actually a few inches longer than it appeared. The additional inches were invisible, which gave him an incredible advantage early on - if one of these attacks got, say, his throat, that would have been the end of the fight.