Say help, then Skeleton Centaurs to Dionysus Argantma, the Centaur Priest, located in the Centaur Town.

Go kill 12 Skeleton Centaurs, located in the basement of Mordenthal's castle, then return to Dionysus Argantma and tell him again Skeleton Centaurs.
Rewards:

• 10,000 XP per level
• Horse Friendship Scroll

You can complete this quest at will.