On the desert, after you defeat the lizard you HAVE to kill for storyline, you can use reveal around there, and you'll find foot prints in the sand. follow the footprints and it'll lead you through a cave. Once you get out, there will be a red tornado, go through that and you'll end up on crossbone isle. If you water it, it'll be just like the boss in the desert, although, I think it's a little weaker.

did you kill the tempest? lizard if you did dont douse it or are talking about the storm lizard beat it then go back up the rocks you climb down and use reavel to see flash ithink he joins withought a fight now go through the cave and go in the res tornado and it will take you to the island