Wizard – “The Triumvirate” [Campaign 6, Mission 3]

I'm also playing through this mission where Markel is hiding. I've already freed Godrick and that elf, I don't remember his name, but I can't find Markel.

There's a castle in the corner of the map (Necropolis) behind the gates.
You approach it, and they ask you if you're ready to fight Markel, but there's a catch. If you attack, you're done.