The Implacable Enemy it is Morally Unambiguous to Oppose
- {Berserkers / Inhibitors}, or lich+undead, or zombies seeking to destroy all life
- {Zerg / The Mist / BUUUUUUUGGGSSS} - biological horrors, traditionally festooned in spikes
- {The Thing / Peter Watts Things} melding that flesh
- A hedonistic Wild Hunt cutting things down for the evils (spiritual / technological / biological origin)
- Doppelgangers / clandestine alien invasion replacing the living
- {Satan} + legions / antediluvian wicked seeking to corrupt all life
- Old gods {Cthulus} with alien or inscrutable morality crushing all life like bugs from indifference or incompatibility
- Clones/forks {quantum thief Gogols} of ultrarich/tech bros/gilded age robber barons/religious fanatics and their semi-lobotomized servant versions of themselves set out to replace everything with themselves
- {Annihilation seed / glimmer or Roadside Picnickers / Vorhh jungle} ~seeking to make weird and alien all life.
- {The Darkest Outcome} changing reality to be 'Weird' and inimical to humanity
- The {Eschaton, Festival} has arrived for the singularity. (opposing least morally unambiguous of these entries unless malevolence is specified - perhaps their culture is steamrolling yours?)
- Archivists / grey goo / journallers seeking to preserve all life in its present state, static, either by scanning, killing, and arranging in dioramas everything that exists, or by being a grey goo that makes habitats (like {Greenfly from Revelation Space} where all life is trapped within, perhaps held in societal and technological stasis)
- Generic genocidaires/fascists or mundane empire
In no particular order. Definitely variation in quality. Obviously, these go farther if combined.
(I use curly brackets to mark direct references)