This mod is a simple but very configurable mod that is designed to work with The Lost Cities (https://minecraft.curseforge.com/projects/the-lost-cities). It makes buildings a lot more challenging by spawning additional and stronger mobs in them. There is a big chance that a building is 'haunted'. If it is haunted then there very strong vanilla mobs will spawn (boosted for damage and health) until the player manages to kill a specific number of them at which point the building is considered 'cleared'. The mobs spawn when the player is in the building but regardless of lighting level so it doesn't help to light up the building (can be disabled with config).
This mod is very configurable. You can control what kind of mobs can spawn this way, how many can spawn, the kind of armor and weapons they get. How strong they are and so on.
Note that this mod is server-side only.
Feel free to use this mod in any modpack you want.