It can be a noun or a verb.
As a noun, generally it's a state of being. What's coming? Doom.
As a verb, it is more of a pronouncement. You are doomed.
It also happens to be the name of a video game, in which it could well be a proper noun.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "twirl" is a verb.
The part of speech for "explicit" is an adjective.
The part of speech for "indefinite" is an adjective.
The part of speech for "lecturer" is a noun.
The part of speech for "breezy" is an adjective.