Kalgrath, also known as the "Night Beast," was a Massassi warrior. Before the Republic/Jedi assault on Yavin 4, the Dark Lord of the Sith Exar Kun placed him in suspended animation within the isolation chambers located under the temples, with the purpose of guarding the moon against any intruders.
A Republic war fleet arrived at the jungle moon of Yavin 4 after Ulic Qel-Droma revealed the location of Exar Kun's base. As the fleet made its appearance, Exar Kun, observing from the top of his temple, commanded Kalgrath to descend into the isolation chambers that Naga Sadow had constructed beneath the temples.
In that location, Exar Kun subjected the mutated Massassi warrior to further alterations. He hardened his already tough hide to the point where it could withstand most standard energy weapons, changed his skin color from crimson to a deep green to provide better camouflage within the catacombs and jungles of Yavin 4, and took away Kalgrath's ability to speak. Using Sith alchemy, Kun induced a deep slumber in Kalgrath and abandoned him there as a guardian. After most of his people were wiped out, and some escaped into unknown regions, Kalgrath would remain in hibernation for thousands of years.
During an assault led by Admiral Amise Griff, a TIE/sa bomber crashed into the isolation chambers, causing Kalgrath's suspended-animation capsule to break open and waking him from his sleep that had lasted for millennia. Believing his people were still alive, and unaware that the Massassi had been killed in Kun's desperate attempt to gain supernatural power, he set out to find them. Instead, Kalgrath came across the Rebel base situated within the ancient temple. Mistaking them for his ancient adversaries, he launched an attack. However, Luke Skywalker and R2-D2 intervened and stopped him.
Skywalker used the Force to establish communication with Kalgrath. Kalgrath managed to regain his composure, and his madness began to subside. Luke offered to assist him in locating his people. Kalgrath was placed aboard a GR-75 medium transport that was programmed to automatically jump to the ancient location of the Sith Empire in hopes of finding any surviving members of his species. One of the transport's random jumps brought the beast back into Alliance space. Using the Force again, Luke explained that the beast would eventually find his former masters by returning to the transport.
Tionne Solusar conducted research into Kalgrath's ultimate fate after Luke established his Jedi academy, and she was able to track the creature down to the deserted world of Ziost before the trail went cold. Researchers on Ziost informed her that they had observed a dark shadow lurking around their camp. Despite this information, it remains uncertain whether this unique creature ever reached the fallen Sith Empire, with only rumors to support Tionne's theory.
Later, on the planet Seidhkona, the Mind-witch S'ybll created an illusion of Kalgrath using Luke Skywalker's memories. Skywalker fled from the illusion, running into some ruins that collapsed on him and rendered him unable to move.
As a result of Exar Kun's alterations, the Night Beast possessed the instinctive ability to shield himself with the Force, rendering him impervious to energy weapons, including all types of blaster fire and even attacks from a lightsaber.
The "Night Beast," who would later be identified as Kalgrath, made his initial appearance in The Night Beast, a comic strip story arc that was written by Archie Goodwin, illustrated by Al Williamson, and published in 1982. His name, Kalgrath, was revealed in the 2001 reference book titled The Essential Guide to Alien Species. The Night Beast bears a resemblance to the Sentinel, a creature that guarded an underground city on the planet Alashan, as depicted in "The Guardian of Forever!" storyline within the comic book series Star Wars Weekly. This storyline was written by Chris Claremont, illustrated by Carmine Infantino and Gene Day, and published in Star Wars Weekly 113 on March 12, 1980.