Kalgrath would remain in hibernation for millennia after all his people were exterminated. With his people eliminated, he would be transformed into a large, green, monster. Thus, he became the Night Beast. In 0 ABY, he was awakened from his millennia-long sleep by a crashed TIE/sa bomber during an Imperial raid. He then went to look for his people believing they were still alive.
Instead, Kalgrath discovered the Rebel base in the ancient temple. Thinking they were his ancient enemies, he attacked them. However, he was stopped by Luke Skywalker and R2-D2.
Skywalker managed to communicate with Kalgrath through the Force. Kalgrath was able to calm himself down, allowing his insanity to fade. Luke agreed to help him to find his people. Kalgrath was put aboard a GR-75 medium transport on auto-pilot to Ziost. The ancient warrior was upset upon his arrival, for he found only a frozen wasteland. It is possible now that he discovered that his people were already killed.
After Luke founded his Jedi academy, Tionne did some research into Kalgrath's fate. She was informed by researchers on Ziost that they had seen a dark shadow creeping around their camp. Despite this, Kalgrath's fate is unknown.