Location: Beishan Village
Petitioner: Yang Kai
Reward: Jadeite Guanyin, Martial Arts Thoughts, 2,000 coins

Once you gain access to the world map you can visit Beishan Village at any time. You are most likely to find this place, however, during the Frozen Corpse in Beishan side quest. Either way, Beishan Village is in the northwest of the map, not a terribly far walk from Pingkang City.

The petitioner, Yang Kai, is found along the village's northbound path. His village (like so many in this game) is plagued with bandits, and there aren't enough men in Beishan to deal with the problem. Yi offers to go kill some bad guys on Yang Kai's behalf.

Your next destination is the Precipice. Leave Beishan and head west a little ways until you see a narrow path through the northern mountains. The Precipice is waiting at the other end. This area is patrolled by a large number of Greenwood Robbers, and they count as bandits. Kill five of them, which should take you two battles. These fellows have over 2,000 HP on average and hit pretty hard, so don't take this quest too lightly.

Once you've beaten five Greenwood Robbers you can return to Yang Kai to end the quest and receive your reward.