Molto was the supreme warrior for the subterranean race known as the Lava Men. Molto was sent to the surface world to investigate, but was enchanted by Loki and began to raise havoc, generating fires wherever he traveled. Eventually he made his way to New York City, where he encountered Thor.
After a struggle, Molto covered Thor in a quick drying Lava which held him in place and prevented him from moving until Thor exerted all his strength into freeing himself. So Molto covered himself with volcanic rock in an effort to crush Thor. Thor instead generated a powerful whirlwind which sent Molto to another volcano and submerged him in it. Thor sealed the volcano, and Molto returned to Subterranea. Having recovered from Loki's manipulation, he informed the King of the Lava Men that the people of the surface world were peaceful and no threat to them.
A short time later, an explosion from deep within the Earth rocked the realm of the Lava Men. The Lava Men learned that the Living Rock would generate explosive force when struck, and that it would continue to grow in size until it exploded. Jinku, the witch doctor of the Lava Men, counseled the King to build a machine and drive the Living Rock to the surface world where it would destroy the surface of the planet instead.
Despite Molto's protests, the King agreed and followed Jinku's recommendations. This brought them into a conflict with the Avengers, which ended when the Avengers duped the Hulk into striking the Living Rock in the one spot that would destroy it. With the threat over, the Lava Men returned to Subterranea.
Some time later, Jinku had a vision in his dreams promising to lead his people to conquest. Unbeknownst to Jinku, this dream was actually caused by They Who Wield Power for their own purposes. Nonetheless, Jinku convinced the King to steal the Mole Man's volcano-activating machine from the Moloids. The machine lacked a power source, so Jinku sought to obtain Mjolnir, the hammer of Thor, as that source.
To this end, he ranted of his plans to destroy humanity in front of Molto, who he knew would inform Thor, causing him to investigate. Jinku followed Molto and blasted him, mortally wounding him, but not killing him instantly. Molto made his way to the surface world where he encountered the Human Torch, whom he initially mistook for an agent of Jinku, but then convinced him to summon Thor. After relating his story to Thor and the Torch, Molto died, and his body disintegrated (as all Lava Men do at death). But his warning enabled Thor and the Torch to stop Jinku's plan.
Above average powers of a Lava Man: Superhuman strength (Class 10) and durability, immunity to heat, able to generate great heat, project ash or lava which can solidify into volcanic rock.
Unusually acute hearing and night vision to enable Lava Men to function more easily in the darkness of Subterranea