Overseer Martin is an NPC in Dishonored
He helped get Corvo to escape from Coldridge Prison but later Martin is captured.
You encounter him in Holger Square. Use the Release Lever to free him.
Later he is found near The Lighthouse Elevator.
Carries the Lighthouse Elevator Key
Dialog after freeing him in Holger Square[edit]
What you're here to do tonight is of the highest importance. We've got to find Emily, so kill Campbell and make it quick. Once it's done, search his body for the journal - his notorious black book - and get out of there. Campbell is meeting with a guard name Curnow, and word from my informant is that Campbell is going to poison him. Maybe you can use that to your advantage.
Alright, I won't be of any help here, so I'll make my own way back to the Hound Pits Pub. If I see Samuel the Boatman. I'll tell him to pick you up in the back yard, behind the office of the High Overseer.
