Does the Disintegrate spell permanently destroy Alyxian the Hunter?

Question

In the last dungeon of the Call of the Netherdeep campaign, there is an NPC named Alyxian the Hunter that has this ability (p. 160):

When Alyxian the Hunter drops to 0 hit points, the golem turns into an immobile vermilion haze that can't be harmed or dispelled. If the spear from this area has been taken into the Heart of Despair, the haze disperses as the golem is destroyed. Otherwise, the haze coalesces and solidifies into a new ruidium body 1 minute later, whereupon Alyxian the Hunter regains all its hit points and resumes the hunt.

The description of the Disintegrate spell says, in part:

A creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 10d6+40 force damage. If this damage reduces the target to 0 hit points, it is disintegrated.

A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell.

Does the Disintegrate spell permanently destroy Alyxian the Hunter?

In my campaigns, I ruled that it did not, in part because it would ruin the dungeon. But I'm sure this will come up many times, so I ask here.

Answer 1

Alyxian the Hunter will persist

There is a rule that specific beats general (PHB, p. 7):

That said, many racial traits, class features, spells, magic items, monster abilities, and other game elements break the general rules in some way, creating an exception to how the rest of the game works. Remember this: If a specific rule contradicts a general rule, the specific rule wins.

Normally, when you have two specific exceptions to a general rule that contradict each other, like that of a monster and that of a spell, this rule does not help so much – because who is to say which of the two is more specific? (Well, the DM is, so it comes down to a DM ruling.)

Here, however, we are dealing not with a normal monster that has special rules what happens when it is reduced to 0 HP, like a Vampire. We are dealing with a unique monster. There is only one Alyxian. I would argue that rules for a unique creature are more specific than rules for a generally applied spell, and so Alyxian's protection should prevail.

The definition of "specific" in the Cambridge Dictionary is:

relating to one thing and not others; particular

Disintegrate's effect relates to any creature it affects. Alyxian's effect relates to only one creature and not others. It is more specific.

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P.S. note that the text of disintegrate has been updated in an erratum. It now reads (emphasis mine):

A creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 10d6 + 40 force damage. The target is disintegrated if this damage leaves it with 0 hit points.

This likely would not be relevant here, but it is in other cases, such as a half-orc's Relentless Endurance.

Answer 2

It's up to you.

One of the most important principles found in the Dungeon Master's Guide, and one I far too often see DMs forget is:

The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.

Sure, by a strict reading of the disintegrate spell, it seems to get the job done - the magic that causes the Hunter to persist does not appear to be true resurrection or wish. But it doesn't have to be that way if it is better for the story you and your players are telling. When I ran a group through Netherdeep, they managed to avoid Alyxian the Hunter through the whole chapter and were able to put him down by bringing the spear into the heart. I never got to have that fight, and they were overjoyed at their successful, combat-lite trek through the dungeon. I didn't make it easy, but they got very creative and earned their way into the Heart of Despair, and got to see their careful planning pay off as Alyxian the Hunter, in a fit of rage, faded into mist.