Always power hungry, and believing himself to be the one cat truly worthy of being king of the Thunderians, Grune was the cat army's fiercest warrior, who alongside his 2nd in command Panthro made a name for himself that was well deserved through combat. On a mission in the mountains in the nest of the spider queen Spidera, Grune, while otherwise unarmed proved just how strong a warrior he was when he broke off one of his own teeth to use as a dagger to slay Spidera. However when the time for a new Thunderian general came, Claudus did not give the title to Grunde or Panthro, choosing Lynx-O instead. This was viewed as an insult to Grune, however he chose to remain silent when he and Panthro were sent to retrieve the Book of Omens. Though Panthro thought it little more then a fools errand, a wild goose chase that was meant to be banishment for the two warriors, Grune saw it as an opportunity, because if he and Panthro were to return the book they would truly be heroes, and more likely to be seen as the true thunderian leaders then Claudus and his sons. While on the quest for the Book of Omens, Grune began hearing a voice urging him closer to the Sand Sea, this was the voice of Mumm-Ra. Soon Grune and Panthro found themselves at the Black Pyramid, inside Grune freed Mumm-Ra, and Panthro was seemingly sent falling to his death within the walls of the Black Pyramid. Alongside Mumm-Ra and the Lizards led by Slithe, Grune invaded Thundera, during the invasion Mumm-Ra disguised as Panthro murdered Claudus and captured Jaga, but Lion-O, his pet Snarf, Tygra, Cheetara and the Wilytwins escaped the fall of Thundera. Grune was sent to pursue the other cats but soon his "old friend" Panthro turned out to be very much alive, having salvaged an old Thundertank from Mumm-Ra's pyramid. Grune had a handful of confrontations with the Thundercats, until eventually being forced into a collapsing portal to the astral plane in the Elephant village by Panthro. As the portal closed Panthro lost both his arms pushing Grune in which was ironically based on the same lesson of being willing to sacrifice anything to survive Grune taught Panthro years ago in the battle against Spidera. The fate of Grune himself, and whether he is alive in the astral plane or dead has yet to be revealed.