Grendel was a wing-clipped hornagaunt and a pet to the administrative droid 8t88.
Before 8t88 acquired him, Grendel was native to the storm-filled world of Tertiary Kesmere. Though the droid was calculating and cruel in other areas, he had a particular affinity for his pet, and Grendel followed his master wherever he was commanded to.
In 5 ABY the droid resided in the Government House of Barons Hed on Sulon, as a dignitary on a personal mission for Jerec, and Grendel was with him. Jerec's majordomo, in a jesting move, assigned the droid to a room of less "pedigree" than what he deserved. Questioned about this, the majordomo bluffed saying that it was practical for his job, and his pet. The droid saw through his anti-robotic bias and crushed the majordomo's head, and fed him to Grendel.
Having made an example of him, the servants assigned a more luxurious room to the droid and his pet, with a throne-like ornamented chair. After his mission was over, Grendel growled and crouched near him, bumping his tail. 8t88 tried to comfort Grendel, seemingly upset because of Jerec's bad manners in their recent communication, pointing out that Jerec, as a "biological", was not much different than a hornagaunt. Right then, Grendel sniffed the air and rose to his feet, seeing Kyle Katarn entering the room. The arrival of Yun however distracted Katarn, giving 8t88 time to flee; he pulled the unwilling Grendel, who left claw marks on the floor, to an elevator to the Dark Tower's roof.
The droid and his pet boarded a Lambda-class T-4a shuttle which flew to the Fuel City to the south, and boarded the Sulon Star where 8t88 was told to collect his payment. Seeing his tail thumping in expectation, 8t88 "allowed" him to come along, to serve as his bodyguard; otherwise he would prefer the animal to stay behind.
Together they reached the wardroom when they found Gorc and Pic expected him. Although 8t88 expected that Grendel would cover his back, he was unable to stop the Brothers of the Sith from decapitating the droid. After that, they kept the droid on a chair, expecting the arrival of Katarn, and Grendel remained in the same compartment.
Indeed, when Katarn sneaked into the Sulon Star looking for 8t88, he felt Grendel's familiar presence through the Force and followed it, expecting to find its master. Right then, the droid's head fell down, and Gorc showed himself to threaten Katarn, and fed the head to Grendel who swallowed it whole. Right then Grendel watched as Gorc attacked Katarn, only to be shot down; Pic followed right after.
Grendel was aroused by the smell of blood and attacked Katarn with a roar, who managed to react quickly with his lightsaber, decapitating the animal above the jaw; its body fell near a locker, opening it and causing parts to fall on the floor. Desperate to recover the information for the Valley of the Jedi in 8t88's memory, Katarn cut Grendel open, sliced the biggest of its three stomachs, and retrieved the droid's head.
Grendel is first described as 8t88's pet in the novella Dark Forces: Rebel Agent, where he is given an active role in his master's story. In the audio drama that adapts the Rebel Agent novella, 8t88 calls Grendel a vornskr.
In the earlier game Star Wars: Jedi Knight: Dark Forces II, of which Rebel Agent is an adaptation, Grendel does not appear, nor is even mentioned. After 8t88 is deactivated, Kyle Katarn finds him, attempts to point his blaster on him, and right then his detached head simply falls on the floor.
However, the game designers at one point may have considered Grendel's inclusion since an unused fully textured model and animations can be found in the archives of the Star Wars: Jedi Knight: Mysteries of the Sith CD.
"Grendel" is the name the great monster and major antagonist of the epic poem Beowulf.