About version 0.941, you have to complete a prerequisite quest in order to unlock this one. Not sure which one is, but once Ao Madoushi (quest) is done, Kill setag quest become available.
Kill Setag is the third optional quest. It is obtained by talking to Halaster in the eastern part of Kalm. The objective is to find and kill Setag, an evil warlock. Setag is located in the southwest corner of the Barrens and is guarded by three abombinations. Upon encountering Setag, Halaster joins the battle and Setag summons four additional abombinations to his aid.
Halaster and Setag are almost of equal strength. If Halaster dies the quest is marked as failed though the game still treats it as completed. Also, Setag cannot summon any additional abombinations later in the fight. He is pretty weak without them. Once Setag is defeated (if Halaster is still alive) Halaster rewards you with 750 EXP and 1250 Gold. He then teleports away and is never to be seen again.
After obtaining this quest, the player may also buy information on Setag from Mae'chen for 190 Gold. This information is only available until you encounter Setag.