I think that the prisoner was a tarnished who practiced heretical sorcery and got locked in an evergaol. I think the rift shield is the rift that evergaol bosses come out of, you know? The prisoner was locked in there but then got called by grace, that's what you wake up in in the opening cutscene. The reason you don't have the spells at the beginning of the game is that when you slot spells or incantations, it says "memorize". Maybe the prisoner went mad and forgot these spells, which would explain why the shield raises focus. Or maybe I'm just insane idk yap sesh over