A gate to Oblivion may be locked or sealed to everyone except Daedra and the portal's creator. If a gate is created and a sigil stone placed on one side the gate will remain open. But if some part of the sigil stone were to be broken off, and cross over the gate, the portal will become a one way path to all but the one who controls the sigil shard.
The shard must be attuned using basic elements, only then can one pass into the gate. Once per basin, in this exact order: Burn, then bury, then drown.