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.
However, said NPC states explicitly that he will not attack Strahd (emphasis mine):
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.
They might try magical compulsion, but
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…
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,
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?