How to buy the abandoned house? And how to unlock golden magical doors with black crystal

[Deadbot704]

So I've seen that you could buy the house in the village but I have no idea how to do that, and Im aware whenever you defeat a boss you get a black key to unlock the next area but it gets consumed so how does one open those other magically sealed black doors in other floors?

[-A12-]

Beat the game, regardless of endings, find an old man standing in front of house door who will be selling the house.

Wear/equip the Witch Ring to open the golden doors with black crystal. To find the witch ring; go to a bonfire just outside of stairwell on floor 24, and use the wrap circle on the right hand side of bonfire, open chest that is in front of drinking pond.