Kaltor

Kaltor was a heroic warrior of the sea who had passed his work ethic to his “little minnow”. But his risk-taking ended up costing him dearly. He was rescuing trapped Atlanteas from a collapsed mine when exposed to a gas of unknown origins. Kaltor started to periodically change into a two-headed monster of giant proportions. In this form he had no control of his actions. The hero would go on mindless rampages. 
 
Unable to stop himself, Kaltor went after the “little minnow” who had become a King of Atlantis. He claimed to have been trying to destroy the monster for some time. But supposedly felt his battle prowess waning while his years advanced. He bound Aquaman with a sacred oath to kill the beast if Kaltor himself could not. A later encounter established that Kaltor had manipulated into killing him. Though honor-bound, Aquaman was reluctant to face his mentor.  
 
When forced to choose between killing Kaltor or witness him killing Mera and Aqualad, Arthur took the first option. He confronted Kaltor with a sword in his hand. When that sword broke, Arthur forged another one out of shell. The beast finally fell to his blows. The beast was death but its form released a live Kaltor, now freed from his shape-shifting curse.