The Architect of Ruins
Creepy human that studied magic with Katles under Carnum.
Sauls and Kaltes were both apprentices under Master Carnum. During their apprenticeship they became rivals, rather than compatriots. He's known around Rasmark as being somewhat of a dark and moody person, which wasn't how he used to be.
Lunnin had asked Wella to keep tabs on Sauls, since Sauls spent a lot of time walking through the Halfling villages to get to a cave down river. When Sauls left Rasmark, Lunnin and the rest decided to visit the cave Sauls frequented. In one of the back caves, there were piles of animal corpses, their blood squeezed out over the floor in a spiral of blood.
Sauls has gained power very quickly, most likely through a demonic pact. The last time the party faced Sauls he had demons under his command. He was also able to disappear at the last second before Federin could complete the finishing blow to end his life.