Monsters in the vicinity of Mardaneus will be killed when you leave, but there were a couple a little farther away that I took out after talking to him, but before taking the portal out.
You get experience for completing the quest, which is dependent on your level (at higher levels you get more experience).
Kiya's
Reputation-list-spoiler doesn't mention this extra bonus, but Mardaneus took out a couple skeletons I could not reach from inside the room, so possibly we both missed it. Have you tried a bow, or a spell such as Lightning or Hell Spikes?
