A finishing taunt would be a good reward.. You learn to use a secondary special attack 'finishing taunt' If you do this and kill an enemy within the next 5 or so seconds.. You get half of their max hp onto your hp and energy. E.g. Monster with 100hp.. You get back 50hp and 50% energy after killing them with a finishing taunt.. No idea if this is relevant or not.. But it'd be a good 100th quest award. ..Some kind of quest to finish the plague storyline would be good?