The Sepulcher, supposedly has some super secret stuff in it the First Ones were all told to stay out of. There isn't anything in there, just stay out of it. Zovaal decided it held all this sweet loot or what not demanded to get in, started a fight to get into it. Got his ass whooped, banished, and is now butt hurt about it. If the writers had balls the Jailer would breach the Sepulcher and find out that it really was empty. Just a pristine empty room, and that all he had done was for jack shit.