Heatblast is a Pyronite (pyro meaning fire and "nite" as in ignite) from the star Pyros. Heatblast is Ben's first transformation, accidentally activated in the pilot episode of the original series, "And Then There Were 10" when Ben was trying to remove the Omnitrix immediately after it first attached to him.
Heatblast is a magma-based life-form that can control fire at will, either projecting it in a wide range of forms or absorbing it into himself. He can fire streams of flame, create fireballs, manifest tornadoes of flame, utilize those tornadoes to carry himself and others short distances, and radiate heat in all directions for a localized explosion. He can also achieve flight by firing at the ground to propel himself like a rocket. Apart from this, Heatblast is also a highly durable alien, able to endure being thrown through buildings without being slowed. When Ben gets a cold, Heatblast's powers are reversed, giving him cryokinetic (cold and ice) powers instead of the normal pyrokinesis (heat and flame). Heatblast's dependency on heat is also his weakness, as his powers can be negated by sufficient amounts of water or similar fire-extinguishing materials and large amounts of water can extinguish him permanently. He is also unable to lower his own body temperature, which prevents him from touching things without burning them, people included.
The Ben's 10 year old version of Heatblast made an appearance in Ben 10: Ultimate Alien episode "The Forge of Creation". His look changed a bit, his rock was redder, and his hands and the spaces between the rocks became a more solid color of yellow. The Omnitrix badge also changed from white to green. Heatblast also reappears in the Ultimatrix in "Vicktor: The Spoils" to defeat King Xarion in Dr. Viktor's body, now he is Heatblast now has a darker skintone, similar to Alan Albright. In the future, Heatblast has two mini volcanoes on his shoulder, also he has become much more muscular and he can fire lava flames and beams.
Heatblast appears as a playable alien form in "Ben 10: Protector of Earth".