It is obtained as a reward for the unmarked quest "Summoning Karstaag." The quest requires locating the skull of Karstaag and placing it on his throne in Castle Karstaag Ruins. This summons the beast, and battle will ensue. Once he has been defeated, Summon Karstaag is received.
"Summon Karstaag to fight for you for 120 seconds. You may only use this ability 3 times, and only while outdoors."
Karstaag will fight alongside the Dragonborn much like the initial battle, however if he is attacked three times he becomes hostile.