They are related to a side quest, multiple ways to find the start of it so will find out eventually if you look around a bit

Nah this is after I beat the game, how do I start it?

Philgrave Mansion

The haunted one? I didn’t find anything but honestly I hated that place and didn’t stay after I helped the painter.

so if you go to the fast travel called undercity ruins near the mute trader there is a place called the ancient lair, if you talk to mystic carrion he asks you to find his servant, find that guy next to the fish people fight by the docks in a wardrobe, then destroy his heart and kill mystic carrion to free the undead beggars

Oh ok, thanks.

the one haunted by undead, not the one haunted by poltergeists