Ketheric and jail problem?

So I get to ketherics castle and I decide that I’m going to go to the jail to free zevlor before fighting ketheric, so I finish the fight down and there’s no prisoners. So I think that’s a little weird and go to fight ketheric, things are going well and I finish phase 1. Well I go to follow him down into the crevasse in the jail and I find… nothing? I kill the hook horrors and go up the cliff and find the boat and everything but there’s no ketheric and there’s no prisoners. What do I do?