The Lava Monster is a character from Samurai Jack. monster made out of lava and stone, which made its first appearance in Episode X: Jack and the Lava Monster.
Centuries ago, possibly even around Jack's time, the Lava Monster was a human warrior and leader of a Norwegian kingdom. He lived a peaceful life with his loved ones until one day when Aku appeared. Try as they might, the warrior and his forces were crushed almost instantly by Aku with only the warrior left to destroy Aku. However; he soon found out that his weapons, which were ordinary, did not even harm Aku who merely laughed at the Warrior's attempt to destroy him before instantly defeating him. Even so; the Warrior unsheathed his sword, his only weapon left, ready to battle Aku for the last time but Aku had different plans for him. To punish the warrior and make him an example of the price of defying him, Aku imprisoned the Warrior within a unbreakable crystal, which made the Warrior immortal thus denying him a warrior's death. Afterwards, Aku made the Warrior watch helplessly as he destroyed his home and killed his loved ones, and then sealed him in the core of a mountain, making him unable to pass on and join his people in Valhalla, the hall of slain heroes and resting place of all great warriors. After countless years of being stuck in the mountain, the warrior eventually learned how to manipulate and mold the earth around him to create a molten rock body-like shell he could use to move and escape his tomb. He then created a perilous cavern and drew heroes to him, hoping that one would be mighty enough to pass through the cavern safely and free him from Aku's curse.
While traveling through a field one day, Samurai Jack began to hear a voice calling out to him. The voice soon led him into a treacherous cavern full of booby traps and the bones of many before him. After successfully reaching the end, the Lava Monster revealed himself and battled Jack. However, seeing the monster's eagerness to fight after managing to injure him, Jack refused and sheathed his sword. The monster begged Jack to continue fighting, then told Jack of his past and his desire to pass on to Valhalla with the only way to reach is to fall in true combat with another warrior. Jack decided to resume fighting the Lava Monster to try and free him. After a fierce battle, Jack defeated him and freed the warrior of his shell and the crystal within it. But as soon as he was freed from the curse, the warrior began to rapidly age then soon after he drops his sword and falls back-down on the floor. Jack hurries to the now aged warrior and attempts to apologize but the warrior tells him to not to worry and asked Jack to give him his sword which he does. The warrior thanked Jack and when he passed on a pair of Valkyries descended in a beam of light to carry the Warrior to Valhalla. Then just as the Valkyries start to disappear with the warrior in the beam of light, Jack looks to see the warrior turning his head towards him and smiling in gratitude with Jack soon giving a little smile back. Before he left the cavern, Jack drew on the warrior's obelisk a picture of the Warrior and the word FREE in stylized runic Norse. When Jack resumed his journey, the warrior could be seen looking down at him from the heavens in the company of the deities of Asgard, with Odin among them.