Swampfire is a Methanosian from the planet Methanos (referring to methane).
Swampfire is another plant alien with a fire-patterned head and root-like feet. He is described as a living swamp. He has superhuman strength, vast regenerative powers, the ability to control plant life through a type of flammable gas, and the ability to shoot fire from his palms, which can also be used to launch his body for flight. The fire is created by igniting methane naturally produced within his body, which causes him to constantly emit a very foul odor. He can also produce seeds that quickly grow into vines that he can control. His regenerative ability allows him to alter his own body, such as tunneling underground in vine form, growing his feet to root himself into the ground, heal the limbs of others, or passing through prison bars. However, Swampfire is more vulnerable than Heatblast because he can be frozen for periods of time, unlike Heatblast, who is completely invulnerable to cold. He appeared in pilot episode of Ben 10 Alien Force, in "Ben 10 Returns: Part 1" after the Omnitrix had reformatted.
As Ultimate Swampfire, he is a tree-like version of himself with blue gooey shells containing an organic, blue-colored napalm kerosene gel-like solution (as confirmed by Dwayne McDuffie), containing a highly flammable substance that Ultimate Swampfire uses to create fire bombs, nova blasts, and blue fire tornadoes. Although Ultimate Swampfire cannot regenerate himself, he is also tougher. Ben used Ultimate Swampfire on the first time in the Ben 10: Alien Force's series finale "The Final Battle: Part 2", to fight Vilgax, and he was the first ultimate form used by Ben.
Swampfire appears as a playable alien form in "Ben 10: Alien Force The Game", "Ben 10 Alien Force: Vilgax Attacks", "Ben 10 Alien Force: The Rise of Hex", and "Ben 10 Ultimate Alien: Cosmic Destruction" with also Ultimate Swampfire.