Haven't seen it suggested yet, so I'll throw out Linda Nagata's Inverted Frontier series. Without giving away too much, explorers on the periphery of a collapsed posthuman civilization launch an expedition back towards its center, and along the way find various eldritch monstrosities -- of human origin and otherwise -- as they try to solve the mystery of the collapse. It's more thriller than horror in tone, but it checks your other boxes quite well.