Hey, i just did the reaper soul shard thing, you need 3 which are located near each of the 3 keepers. once you fight a keeper, look around until u see one of the huge floating soul gem looking thing (the one that steals your life unless you eat a soul husk). The closest one to each of the keepers has a chest that gives u one of the shards. Once you get three, go to the room with the soul reaper receptacle (which is roughly southeast of the keeper closest to jiub, and the wall halfway between valerica and the entrance to the soulcairn). Activate the receptacle and it spawns a reaper and about 6 bonemen, once you kill the reaper he gives 3 black soul gems not filled. Not the greatest reward but as your getting the reaper shards, the chests that contain those, drop some nice goodies anyways. Have fun!!!
P.S. the gate didnt uncover but maybe there is something else to do that i didnt see pls respond if u find out.