When playing several quests with the same heroes, they level up and gain advantages at the beginning of a new quest.
The overlord also gains something to average out.
In the yellow box on the first page of the Descent JitD quest it is stated that the overlord receives e.g. +1 card, +1 threat token per player.
Tell me now, is this per player (so five in our case) or per hero (four)