beat corpse by going on the alt floors and garbing 2 knife pieces in downpour 2 and mines 2 then go in to mausoleum and beat mom, then use the full knife you made with knife piece 1 and 2 on the blocked red door way and beat mom's heart, then do down to corpse and go and beat the floors. on the final floor (corpse 2) you will find a hole on the ground in the boss room, go in it and fight "mother" was also known as "witness" once you beat mother go in the chest and win,
tip
if you are good at it, use the forgotten to beat mother and to learn her attack patterns, use the soul and hide behind the skeleton
(also you may not want to unlock Jacob and Esau, they are hard)