Lava Lord was the original ruler of the underworld where Skullmaster was exiled, Lava Lord was sealed in solid rock by Skullmaster after the two battled for dominance. He was then freed when Max and crew destroyed the Crystal of Souls. Using his loyal Lava Beasts and Magnus - his lava-powered war machine - Lava Lord then made a bid for power again, plunging the underworld into chaos. Max later enlisted his help when attempting to stop Skullmaster from resetting time, even though Lava Lord has no love for the Mighty One, either.
In truth Lava Lord wasn't really a villain, having no desire to conquer or subjugate realms outside the center of the Earth. He was, however, extremely vengeful toward anyone who earned his ire.