Rhiahn is an unusual creature with a large, disc-shaped head with huge jaws. He has yellowish-green skin and clawed hands. Rhiahn is capable of both bipedal and quadrupedal stances, alternating between them to best suit the battle he is currently involved in. Rhiahn also has a set of blades on his back called 'Bio Blades', that allow him to fly and can also be used as weapons.
Another of the Mega Monsters employed by the alien warlords from Mega in their attempt to conquer the Earth. Of all of their champions, Rhiahn was perhaps the most bizarre, and the most deadly. During the mass battle in Salt Lake City, Rhiahn repeatedly attacked Godzilla, carrying the immense reptile clenched in his gigantic jaws. More, it was Rhiahn who removed Red Ronin from the conflict, severing the robot's head with his anterior bio-blades and leaving Godzilla alone to face all three Mega Monsters.
Of all the Mega Monsters, Rhiahn is the most direct, preferring to strike an enemy with his gigantic jaws and clawed arms in a more obvious manner than the stalking and lurking tactics of Triax and Krollar. Rhiahn is easily driven off by injury, he lacks the extreme durability of Triax in this regard, but the monster is quick to recover and swift to return to the attack. Rhiahn is extremely tenacious in his attacks and once an enemy has provoked him, the alien monster will fight back relentlessly until one of them has been vanquished.
- Like other Mega Monsters, Rhiahn can survive in the vacuum of space.
- Rhiahn's powerful jaws can be used to bite opponents. Rhiahn's grip is so powerful that it can only be released on his own accord.
- Rhiahn can use his Bio Blades to fly. The blades function in a similar manner to rotor blades, allowing Rhiahn to fly and hover like a helicopter.
- Rhiahn can use his Bio Blades as weapons to slash opponents.