For this faction spotlight it will be morbid and interesting. For its the main mechanic is that your Cities will produce a rotten flesh like substance that looks oddly like a universe on the verge of collapse that will expand and this artificial dominion will have an wide range from adverse affects such as stripping enemies of all militaristic value and blessing will have a 3/10 chance that it will back fire
Now onto units.
For starters your villagers are replaced with. Cultists who are unable too gather as fast as other but have magical attacks and you can promote them to be a cult leader of a specific god in your pantheon and for Myth units just look up any such creature from the settings and mythos and it works.
For the monument's you have the Mountains of Madness and the Arkham Asylum for a prison and find some use of the R'lyeh house.
For the titanic creature it has to be something like the. Cthulhu from smite as a inspiration because that monster is the mascot and even made it in to SCP foundation.