The Lava Men were originally Gortokians who were the first race of Subterraneans created by the Deviants as slaves. After the Gortokians and their leader, Gor-tok, rebelled against the Deviants, the Gortokians freed themselves from bondage to form their own community.
Later, a faction of Gortokians worshiped a demon called Cha'sa'dra. This faction of Gortokians were transformed by Cha'sa'dra into the Lava Men who left the Gortokian community.
A lone Lava Man called Molto first appeared on the surface world before his brethren in Journey into Mystery #97. Then the Lava Men appeared together in Avengers #5.