Eventually , his jealousy towards the king was used by Mumm - Ra to convince Grune to become his follower and serve a key role in Thundera 's downfall .
Grune felt betrayed and drove himself mad with his own paranoia .
Grune 's jealousy towards the king was used by Mumm - Ra to Grune Grune to become his follower and serve a key role in Thundera 's downfall .
Grune felt betrayed and drove himself mad with his own paranoia .