Quest is simple, go outside of the cave "Crystal Depths" and kill the human mobs until they drop the portal keystone. Once you have that go to 4799, 1219. That is the cultist camp, on the west side of the camp is a purple portal. Ride through the portal and you will be teleported to the proper spot to kill the Frostgale Thundergust. There are about 3 or 4 of the mobs in the area.