"Summon Son of Crawmerax" is an objective in the mission Fun, Sun, and Guns in Sir Hammerlock vs. The Son of Crawmerax.
Walkthrough
Head over to the East and drop off the ledge to the area below. You will land in a massive sandy arena. Turn to the West and head over to the Summoning Wheel. Interact with it and you will call forward The Son of Crawmerax the Invincible.