In Curse of Strahd, there is an NPC the PC's may encounter whose weapon explicitly does more damage when he uses it to attack Strahd.
Vladamir Horngaard: +2 Greatsword. Damage: 4d6+4 slashing; Against Strahd, Vladimir deals an extra 14 (4d6) slashing damage with this weapon.
However, said NPC states explicitly that he will not attack Strahd (emphasis mine):
“If you have come to free this land from the creature that feasts on the blood of the innocent, know this: There is no monster I hate more than Strahd von Zarovich. He slew Argynvost, broke the life of the knight I loved, and destroyed the valiant order to which I devoted my life, but Strahd has already died once. He can’t be allowed to die again. Instead, he must suffer eternally in a hell of his own creation, from which he can never escape. Whatever can be done to bring him misery and unrest, I will do, but I will destroy anyone who tries to end his torment.”
The module says that the players may want to take his weapon; but its description explicitly states that the damage bonus applies when he is wielding it.
The players might try to persuade him to ally against Strahd, but he will refuse.
Vladimir fights in self-defense. He also rises from his throne and attacks if the characters fail to heed his warning and press him for help destroying Strahd.
They might try magical compulsion, but
As an "undead", Vladimir is not affected by spells that target "humanoids". He also has Condition Immunity: Charm.
None of Strahd's three goals give him a reason to go to the NPC's location. Unless the players visit the NPC's location, he will not leave. I suppose players could get him to venture to Strahd's castle...
If the players destroy Vladamir's current body, his spirit will find another body to inhabit. A normal revenant would then seek out its killers, but the module states that the revenants in Barovia are kept trapped there regardless, so it is not clear whether Vladimir's newest incarnation would seek the PC's out or would return to his original location to brood on his hatred for them and Strahd. If a DM ruled that Vladimir's new incarnation seeks out the PC's, clever players might slay him and then enter Castle Ravenloft to force him to enter as well and confront Strahd...
but even if the two are brought together, neither has a reason to attack the other.
The only event that will remove the thing preventing him from attacking Strahd,
lighting the beacon to end the hate that keeps him bound to Barovia,
will also end his life.
Any way I look at it, I can't see a way that this NPC would ever attack Strahd, so what is the relevance of his weapon doing extra damage to Strahd when he wields it?