The House That is a God

Like a genius loci, mostly.

Adherents that sleep in the house lose 1 hp from their max each time they wake, to a maximum of 5 lost. These go to a pool of hp the house can use to improve itself, gain abilities, grant spells to clerics, empower warlocks.

The hp is recovered by leaving for a night if in the first 5 nights, then can't be recovered after. Cultists often capture and chain up folks to feed the house. Some die.

The house's consciousness is deific. Given sufficient time it can expand to be something like Malfeas, a plane or demi-plane with a consciousness.

It can spend 1 hp to restore 1 hp of its own, up to 10 hp per night.

It can spend 1 hp per square foot of space to add onto itself (a 5x5 square is 25 hp, thus).

It can spend 100 hp per spell level to cast a permanent spell within itself, or double that somewhere in its grounds (e.g., guards and wards, or silent image, or something). Some spells are more appropriate 1/day (like teleport).

It can spend 100 hp per CR to summon or create a creature inside itself.

It can spend 100 hp to improve one of its mental stats by 1.

It starts with a small home, 600 or 800 sq ft; standard materials including AC, DCs, and hp; INT 3, WIS 3, and CHA 3.