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The Iron Golem has the following feature:
Immutable Form. The golem is immune to any spell or effect that would alter its form.
Does this feature protect the Golem from the effects of the disintegrate spell?
I found a few discussions indicating that it would work, but nobody talking about this specific feature of the iron golem.
The disintegrate spell doesn't actually do anything to alter the golem's form until the golem is already dead.
There is nothing in the language of disintegrate that suggests that there are certain creature types that are immune to its final effect.
A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust.
Because of that, it does suggest that the spell works on all types.
The terms of this feature seem to suggest that its immutable form is more to protect it against polymorph spells and other form-changing magic/abilities.
The turning to ash upon death from disintegrate isn't really about changing its form - it's about destroying it's body once killed.
While there could be a case that destruction of the body falls under Immutable Form, it makes more sense to look at the ability as something that prevents it from being converted into a different creature and not something that prevents its total destruction upon death. While Crawford's ruling is not RAW, I think the interpretation of destruction not being equivalent to shape change makes sense and his tweet below supports that.
A golem, like other constructs, is a creature. In fact, all the monsters in the Monster Manual are creatures. When a spell, such as disintegrate, says it does something to creatures, the spell means a creature of any type, unless the spell makes exceptions.
disclaimer I myself couldn't find info stating that monsters lose features upon death so please correct me if I am wrong.
Based off of the Iron Golem's stat block, it does not specify that once it hits 0hp it no longer retains its transmutable trait.
Thus, even though Disintegrate states that it turns a creature into dust upon bringing them to 0hp.
Exceptions Supersede General Rules (p5 XGE)
The game also includes elements-class features, spells, magic items, monster abilities, and the like- that sometimes contradict a general rule. When an exception and general rule disagree the exception wins.
In this case Immutable Form would be the exception to Disintegrate's general rule.
