The evil Grune was once a great ThunderCat but was corrupted by the forces of evil and his lust for ultimate power. He wrecked havoc and chaos throughout the galaxy, until Jaga, who was once his most loyal friend and is now the greatest threat to his power, defeated him and banished him from Thundera forever by setting him adrift in space, where after many years he finally crashed on Third Earth. Grune then spent most of his time looting and destroying everyone and everything on Third Earth. Grune's spirit was seized and placed in an underground sealed tomb. Many years later after being freed from his tomb by Bulkin's, Grune came back in ghostly form to finally settle his dispute with Jaga.
("Grune the Destroyer" in the original series, in the 2011 series) is a former Thundercat noble who was once a good friend to Jaga the Wise on Thundera. However, his lust for power and greed turned him renegade against his fellow ThunderCats, forcing Jaga to fight him in a titanic battle that went on for days. Grune was defeated by Jaga, then banished and sent into exile in space. He possesses immense super-human strength. Eventually, his spacecraft landed on Third Earth and he began terrorizing its natives until, many years later, he was defeated (by unknown means) and sealed away inside a tomb (which was covered with a stone slab and a tree to prevent others from opening it). At some point, Grune lost one of his fangs. A tablet held the seal in place, until centuries later, a pair of Bolkins accidentally released his spirit. Once freed, the ghost of Grune seeks out the ThunderCats, attacking them to take out Jaga, whose spirit ultimately defeats him using both Lion-O’s strength and the Eye of Thundera. However, Grune would return later when summoned by Mumm-Ra to stop Jaga’s spirit from obtaining the Star of Thundera (which was needed to help free three more Thunderians — Pumyra, Ben-Gali, and Lynx-O — from the Mutant prisons at Fire-Rock Mountain). When the Thundercats return to New Thundera, Grune is resurrected by Mumm-Ra. Grune initially succeeds in defeating Lion-O, breaking the Sword of Omens and chaining Lion-O and Cheetara up on the Churning Rocks. However, Snarf gathers the pieces of the sword and while attempting to take them to Ben-Gali for repair, he is captured by Char, who repairs the sword and makes it even more powerful, wanting to keep it for himself. Sensing the sword is repaired, Lion-O calls for it and uses it to free himself and Cheetara. Grune attempts to defeat Lion-O, but fails and is banished back to Third Earth when Lion-O activates the ThunderCat signal.
In the 2011 series, Grune is one of Claudus' trusted generals who were best friends with Panthro in the past while confiding in him his desire to rise up the ranks. During one of their missions, Grune ripped his sabertooth off to save Panthro from Spidera. When Grune and Panthro sparred to prove themselves to get a promotion from Claudus, Claudus instead promoted Lynx-O in general and assigned Grune and Panthro to find the Book of Omens. Grune felt betrayed and drove himself mad with his own paranoia. Eventually, his jealousy towards the king was exercised by Mumm-Ra to convince Grune to become his follower and serve a key role in Thundera's downfall. However, Grune also intended to betray Mumm-Ra in the long run prior to leading the attack on the Elephants' village to obtain the spirit stone there resulting with Grune getting sucked into the Astral Plane thanks to Panthro (who lost his arms in the process).