This quest is initiated by examining the item of the same name. This item is occasionally given as a reward for examining a Scrying Stone inside of the lights in that have been randomly appearing in the world. Everfrost seems to be the best place to find these.
Once you examine this item you will have 5 minutes to kill 10 centaurs. Any centaur will do, but at the moment the only reasonable centaur population is in the Thundering Steppes.