The Stygian

Overview

The Stygian is a giant enemy worm in the Darksiders game. It is the third of The Destroyer's champions. Its home is the Ashlands, which is a surface upon which War has difficulty moving (he can't run, only walk). The worm has the ability to traverse this surface without issue, and it can also burrow underneath it, disappearing entirely.
 

Battle Versus War (spoilers)

When War encounters The Stygian, he is being held captive by enemy troops. The worm is sticking straight out of the ground, with chains holding him to nearby columns. He has a large metal hood over his head, covering his teeth, but also covering his convenient weak spot. Shortly after arrival, The Stygian manages to break one of the chains, and all the others fall in short order. He crushes some of the enemy troops then starts traveling around the arena.
 
War must summon the recently acquired Ruin, his trusty steed, to traverse the ash and keep up pace with the Stygian. While riding alongside the worm, War must attack the metal hood covering The Sygian's head. Once the locks on both sides are broken, the hood comes off. With the hood now gone, The Stygian is free to try to gobble up War by chasing him. When he opens his mouth to roar, he exposes a glowing orange object in his mouth, much like a uvula, which is his weak spot. War needs to aim and fire at this spot with Mercy, his gun, while continuing to ride Ruin.
 
Periodically, after enough damage has been caused, The Stygian will retire to the ash and two lesser worms will appear out of the ground (at least on the Normal difficulty setting). War must dispose of these to allow The Stygian to reappear. After several instances of doing damage to The Stygian's weak spot and killing lesser beings, an instant kill icon will appear above his head. War must ride Ruin into the creature's mouth and hit the proper button. Doing so will show a cut scene where War rips open The Stygian and removes its heart to return to Samael.