Obsidius' final name comes from obsidian, a type of volcanic glass, and he was chosen by fans in an IGN poll after he was voted into the game. Other names that fans could have chosen include Pyrodorah, Kazango, Dotoryo, and Volcanis, Magmouth being Obsidius' original working name.
Originally, according to Art Director Robert Caracol, "Magmouth was a combination of two separate concepts. The first was a rocky, craggy creature that was basically solid rock. The second was more akin to flowing magma. In the end, we came up with a blending of the two ideas, arriving at a monster that was a walking volcano. It possessed the hardness and sharpness of solid granite but could also attack using the molten fluid from within its body." Obsidius' design did not change much from there. Its colors range from a dark brown to charcoal black, and then its magma ranges from yellow to orange. It has three horns at the back of its head, no eyes, no mouth features—other than a hole similar to a throat where magma can be expelled from—and no real features other than rocky skin and thin lines of magma where articulation is required, like the joints of the arms, legs, neck, tail, and feet.
Obsidius is made of volcanic rock, making him resistant to many attacks. He is immune to lava, and so can run around in the lava pools on the Seattle arena in the game. He can curl himself into a ball and roll around at a high speeds. Many of Obsidius' attacks are similar to a gorilla and Orga, with his huge arms and and flexible legs playing a huge role in his fighting style. Obsidius also has the ability to spit out burning lava (this attack is known as "Inferno") and burrow underground.
- Godzilla: Unleashed (Wii and PlayStation 2 versions)
"Although most of the crystals deposited on Earth by the meteor showers (sent by SpaceGodzilla in order to escape from his prison in Godzilla: Save the Earth) embedded themselves in the surface, some of them managed to punch through the earth's crust and enter the lithosphere. There, brought together by the magma flows from within the earth, a sentient creature was brought to life by the accumulated power of the crystals and the raw, ferrous materials of the earth. Obsidius emerged from a volcano onto the surface world only recently in conjunction with fellow mutant Krystalak, and thus far has resisted all attempts at communication. His virtually impenetrable mineral exterior shields him against both physical and energy assaults. He advances single-mindedly toward the most unstable crystal formations on the surface, and as such should be considered extremely dangerous."
- Obsidius managed to beat Lightning Bug, The Visitor and Fire Lion in an IGN poll to get into Godzilla: Unleashed.
- Obsidius's roars are stock sound effects preformed by Frank Welker for The Land Before Time, though Simon Strange has stressed that this was not done deliberately.
- Obsidius is one of the few monsters that do not have eyes.