you and some guys, hopefully an alliance, form up by the outpost in west sartabarta, and one person will trade that item to the guard.
then waves upon waves of yagudos will spawn and you will have to kill all of them without letting the NPCs helping you die.
upon completion of the task you get some prizes like Garrison gear (ie. Sallet, Tunica, so on), and mannequin stuff, there is also other stuff that is obtainable from garrison
in the info, it says that this yag is a blm then why does it drop whm spells? does that mean priests will drop stonega for me than? (hehe sarcasm ^_^) anyways, just found it kinda weird....
Drop down the hole (in the spiky grass) at H-7 on the map and follow the tunnel to a small plateau. stay on top of this and hug the left wall, after you pass through to the next area hug the right wall & go through the odd beehive-looking structure, you'll start running into them from there onwards - drops a key for the Kazham airship pass btw.