I didn’t see any dedicated monster brainstorming posts that were in-date so here’s this. Let me get started with a few end-game-ish ideas.
The Crimson/Broiling/Blood God. (I’m not so good with naming)
Kilometer tall behemoth of flesh and blob, insides superheated to the point of boiling and raising the temperature of it’s immediate area well above live-able. As well as boiling rain that falls around it constantly. Could have some variants with acidic blood or Mycus spores? Would be simply unkillable by the player with the only reaction possible being escape. It would be very fast being that each and every step it takes are hundreds of meters. Would spawn other enemies from itself such as flesh-raptors (which in lore would be there to retrieve biomass to give to the colossal thing to feed it’s continued existence)
Ruby Deep: A horrifying amalgamation of oceanic life. Oceanic life, and unlife, will ALWAYS be capable of sustaining far, far larger bodies than that of terrestrial beings. Such that this cataclysm in and of itself dwarfs the Crimson God and causes a far greater meteorological phenomenon, The oceans around it boil red from the immense internal temperatures of the unknowable mass of flesh and carnage. (for you’d die long before you ever came close enough to witness it first hand) The constant broiling of the ocean would create a vortex centered on the beast. The vortex would pull everything within the ocean towards the monster and the constant flow of superheated air and water vapor into the sky would create a permanent tropical storm. (I don’t think living on the beach or, by god, an Island is such a good idea anymore.)
The blank(again not good with names) mobile factory. A massive machine god of war that produces mini-tanks, chicken walkers, and other war machines at a dizzying speed. The mechanical nightmare itself capable of lobbing artillery shells, laser salvos, and if you’re truly unlucky (you were actually able to damage it) nuclear warheads at you. This machine would have been Humanities guardian, a staunch protector at the end of days. But alas, the few people who were capable of controlling it have all perished due to one circumstance or another and you are not entirely human anymore. As such, you must be purged.
Here’s my three end game style enemies that could; be how you died.