The Corpse (したい) effect turns Sabitsuki into a purple colored corpse. Like many other effects, it is entirely cosmetic and does not give Sabitsuki any powers.
The Corpse effect is found in the Prison by going into the only room there and touching the corpse in the dark corridor between two prison walls.
Down in the nexus, go southeast when in the Ghost World and you'll see two tan colored pillars. Go through them, then go directly right till you reach the Underwater World. Then go up till you reach a large tan colored building. Enter it, and and follow these instructions: right, up, up, left, left, right, up and you should reach a dark area. The dark area has a very clear path. Follow the walls, and when you reach the split paths, choose the north one. Once you enter, go to the top right and enter the door. You should be in the prison. Look around for a little bit until you reach a guillotine (be careful not to click it! It'll teleport to a place where you can't come back!).
By going east of the guillotine with the grim reapers, and slightly up, you should find a door. Go all the way up (this is not a glitch, though it seems to be) until you find a corpse. Interact with it and you'll get the effect!