I really need help with the same thing. I've collected three souls, and the 4th one alludes me. Unfortunately, I started Bridge Ablaze before all the others. Now, all the others are finished and I'm still here. I do not have the key. I have no idea where the key is. It's not on any bodies. There is no longer any loot around. I don't have it either. I've already talked to the priest, so unfortunately I can't do the Betrayal thing. I am at a loss. I don't know what to do anymore.