Triple Regis (Triple Trouble but it’s Regigigas Vs the dead Regi trio (plus one.))

As Regigigas pursues an enemy, who was long banished before this, he finds himself in a cave. Darkness surrounds him. He uses flash to reveal a terrifying sight.
Regice, who’s body was supposedly never supposed to melt, melting. It’s cry is weak and full of pain.
Registeel, a titan of metal, who’s supposed to be oxidization resistant, now just a rusting shell of what it once was. It bleeps a weak bleep.
Regirock, the titan of stone, who’s stone body was supposed to be resistant to erosion, now eroding. It’s eyes moved down to it’s chest as it weakly says it’s name.
Regigigas is terrified as to what it’s children has become. But at the center of it all, a Regi of darkness. A Regi of pain. The cause of the decaying Regi trio.
Regidoom.
Regidoom’s head sticks out of a portal, the stones on it’s mouth form what looked like teeth, the ice on it’s head a lavender hue, looking like crystals. It’s eyes are glowing red with what once were electronics that belonged to Registeel. A true beast. A vessel of vengeance.
Regigigas now has only one option, to fight it’s children, and take revenge on Regidoom for destroying it’s children.