The Ruined Dragon is a huge grayish black dragon. He's rather thin and features two rather long legs culminating in what appears to be three-toed feet, as well as two long arms culminating in hands having three fingers and a thumb, with white sharp claws. His wings are huge with respect to the rest of the body and features five fingers, with two additional claws on the top, one at the carpometacarpal joint, the other presumably at the metacarpophalangeal joint of the longest finger.
The patagia are dark purple at the top, gradually becoming red at the bottom. The body is fully covered by big scales, while its lower part until the head is covered by bigger plates and has a lighter pink color. The Ruined Dragon has a long neck and a head with a long muzzle.
His mouth has gums fully covered by teeth and features a long and thin tongue, curved at the tip. His eyes are round and feature a black sclera, irises which are yellow with a red border and black vertical slit pupils. He features on the back of each side of the head two horns pointing backward which are segmented and doubly curved, with the upper horn being bigger than the lower one, and in general the whole head is covered by smaller horns, two of them have the shape of teeth and are found almost at the end of the muzzle.
The head also features a peculiar segmentation forming vertical bands which are more visible on the lower part of the muzzle. The upper tip of the muzzle is slightly protruding and features two relatively small nostrils.
When first meeting him, the Ruined Dragon is seen wearing a golden crown featuring the shape of Bowser's head on it, as well as various horns and a black cloth with golden motifs similar to the ones seen in Bowser's Kingdom, which is kept into place by chains that are fixed to the dragon's head through some golden sword-shaped pins, identical to the sword-shaped stakes found in Crumbleden. After being defeated, the Ruined Dragon no longer wears said crown.
Compared to other dragons (and most creatures in general) in the Mario franchise, the Ruined Dragon has a more "realistic" design, with detailed textures and little cartoonish exaggeration, just like the Ruined Kingdom he's found in.
While traveling to Bowser's Kingdom on the Odyssey, Bowser is shown standing on the dragon's head, ordering him to attack through a powerful magenta lightning bolt that damages the Odyssey heavily and forces it to land on Crumbleden.
There, he's met on the ruins of the tower and attacks Mario through two types of attacks: the first one is magenta lightning bolts from his mouth that hits the ground in front of him, generating various rotating spiked wheels made of electricity that are able to slightly change direction to chase Mario. The more the Ruined Dragon is hit, the more spiked wheels attack through this method.
The second consists of electrifying his head and slamming the ground with it, generating various magenta electrical streams that diffuse in all directions. Like the spiked wheels, the more the dragon takes damage, the denser the streams are.
A few seconds after the second attack, the head stops being electrified and the dragon is forced to rest, allowing Mario to jump on his head and use Cappy to remove the sword-shaped pins on his head that keep the crown in place; sometimes removing a pin might make a dark violet Burrbo come out. When Mario removes all of the pins, the crown disappears and Mario is able to ground pound the dragon's head. Every time he's hit, the crown regenerates and the number of pins keeping it in place increases. Once he's hit for the third time, he faints and drops a Multi Moon.
The rematch in Peach's Castle is mostly the same, but the streams of electricity the dragon creates after slamming the ground with his head now appear in a vertical zigzag pattern, meaning Mario must jump over some areas and run under others. Also, the battlefield is icy, giving Mario less traction.
Unlike other bosses, he doesn't explode after being defeated and instead rests hanging on the tower of Crumbleden. If Mario gets on the plaza on top of it, he follows Mario with his eyes and closes them if Mario throws Cappy at them, and can even be talked to, though his only dialogue is "so tired" spaced out by ellipses. After activating the Moon Rock, a Power Moon can also be obtained by throwing Cappy at the tip of his upper right horn.