Why would this NPC in Curse of Strahd ever attack Strahd?

Question

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?

Answer 1

It is unclear where the extra damage comes from

Dealing additional damage in this way is perfectly normal for creatures of this type as seen in their basic statblock:

Revenant

[...]

Fist... If the target is a creature against which the revenant has sworn vengeance, the target takes an extra 14 (4d6) bludgeoning damage.

It is fairly clear who the NPC's adversary is:

Enemies of Strahd. Vladimir found himself fighting Strahd’s armies time and again as they swept across the land... With [one of the battles] won, Strahd surrounded Argynvostholt. Rather than cower in his lair, Argynvost emerged and battled Strahd’s armies to the bitter end.

Deadly Vengeance. Unwilling to accept his failure, Vladimir returned as a revenant. So great was his hatred of Strahd and his thirst for vengeance that those feelings fueled the spirits of many of his fellow knights — including Godfrey — to come back as revenants as well. Vladimir continued to wage the hopeless war, even as Strahd tightened his grip on the valley.

However, only this NPC applies the damage in this specific way, as the other NPCs of this type do not get this benefit:

The revenants, if still active, are blinded by their hate and attack the characters on sight, seeking to drive them out of Argynvostholt. As an action, a revenant attacks twice with its longsword, wielding the weapon with both hands and dealing 15 (2d10 + 4) slashing damage on a hit.

That being said, the damage may still come up as shown below:

Not every fight is to the death

There may be instances where the NPC would have to fight Strahd with no intention of killing him. In these cases, his weapon is all that more effective. Some of these instances could include (but I'm sure there are others):

• Defending himself against Strahd if he were to attack him in his domain.
• Preventing Strahd from escaping in some other way if the NPC believes the escape method is credible.
• Any combat that does fit into his direct motives of bringing misery to the adversary.

Nothing explicitly forbids a party from compelling him

A highly proficient effort could be made to convince the NPC to abandon his original mindset and help the party against Strahd.

A DM could rule that this is outside the realm of possibility for the party, but equally valid is simply assigning a particularly high DC to any relevant ability checks. The Basic Rules recommends a DC of 30 for "Nearly Impossible" efforts, and this is where I personally would assign the DC.

Answer 2

The answers are in the first block of text:

Whatever can be done to bring him misery and unrest, I will do, but I will destroy anyone who tries to end his torment.

Because the former is open-ended and because of the focus of the rest of the book, there is no answer as to when the sword would come into play. Because of the latter, the character exists as an obstacle for the PC - one who is not an ally of Strahd, and fits well with the tragic aspect of Ravenloft.

Wrapping it up:

It is not hard to imagine that there might be a circumstance where Vladimir sees an opportunity to make Strahd more miserable. For instance, if Strahd directly opposes Vladimir, Vladimir's damage bonus becomes very useful. Vladimir might use it not to destroy Strahd, but rather to overpower him or drive him away - to achieve whatever Vladimir thinks will make Strahd more miserable.