After many years of peace Grune hungered for battle and power. He considered the Thunderians unworthy of the service of the ThunderCats and sought to rule them as a God. Grune attempted a military coup that was faced down by the mighty Jaga. The two fought for hours and eventually Grune was defeated. Using the Sword of Omens, Jaga encased Grune in a crystalline pod and banished him into space.
Grune reached Third Earth where he looted and dispensed terror across the planet, gaining the name of Grune the Destroyer. He was eventually captured and sealed in a tomb where he remained for centuries until he was released by two Bolkins, Bundun and Hurrick. The Bolkins entered the tomb during a powerful thunderstorm in the hope of finding treasure. In doing so, they broke the magic-seal and release Grune in spirit form. He quickly found the Cats' Lair where he drained the Thundrillium and grew to a giant size. Grune then located a source of Thundrainium and forged a mace from it.
Grune returned to the Cats' Lair where he hoped to gain revenge against Jaga. Lion-O called on the help of Jaga using the Sword of Omens and the two battled again. When Jaga lost his sword, Lion-O threw him the Sword of Omens. Grune's Thundrainium mace weakened Jaga and Lion-O gave him his strength and eventually defeated his old friend. When Grune offered Jaga his hand in friendship, he took the Sword of Omens from Jaga's belt. The Sword rejected Grune and he disappeared.
However this was not the end of Grune as he was revived by Mumm-Ra twice, both times in an attempt to stop the Thundercats.
He appears in the wildstorm comic "Thundercats: Reclaiming fifth issue" and "Thundercats Origins: Villains and Heroes".
- In the Original series, Grune is mainly encountered in spirit form. For the New series, he was alive until Between Brothers.
- Despite being a Thunderian, Grune has been shown to be completely immune to Thundrainium, but how this is possible has never been revealed.
- Since Grune's great-grandfather was once Lord of the Thundercats, that would mean Grune and Lion-O are related.