Is there a short cut to use the crystal skull (summon creature). The game does not really do a very good job of describing this feature at all. I just sort of stumble upon it by accident when examining the skull.
You can assign the crystal skull to a hotkey, use it from the inventory or (in the Xbox version) pause the game and hit a shoulder button to bring up an item/skill menu to use it. I don't know if the PC version has a dedicated hotkey for it.