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.