The Commandant
Nulgath's officer and Dage's mentor
Nulgath assigned the Commandant to teach and test Dage, and over the years they became old friends. The Commandant explained that his master's contracts bound souls while allowing their holders free will if the agreements remained intact. He believed a servant might escape by exceeding Nulgath's strength when the contract was made, though he doubted anyone could surpass his master.
When Dage gathered willing defectors for a rebellion, the Commandant understood their choice but remained loyal to Nulgath, hoping Dage might yet be forgiven. He refused to abandon his master and was defeated by his former student. Dage afterward said he could not part with the Commandant's strength.
Sources
Dage the Lich's dialogue · The Commandant's quests · Fiend Past map