Dethal was a Fremennik, who lived during the Fifth Age, travelling all around from a place to another. He decided to abandon his Fremennik lifestyle after a sea voyage that went wrong, and became an explorer.
Eventually he turned into a vigilante and started his one-man crusade against the corrupt White Knights residing in Falador. He started wearing many different dark outfits and used chemicals to make it look like his eyes were glowing red under his helmets and masks. That, combined with other innovative scare tactics, led the White Knights to believe that an immortal demon was hunting them down and killing them one by one for their sins.
In his final battle he confronted the leader of the corrupt White Knights, Sir Lethipus. After severely wounding Dethal, Sir Lethipus realized that his opponent was just a mortal man. The two of them fought fiercely, but it was Dethal who stood victorius in the end.
Many believe that the injuries Dethal suffered during that night were fatal, but his body was never found. And after his disappearence, many people have claimed that when investigating a scream in the midst of night, they saw red eyes looking at them briefly from the darkness, before they discovered the body of a criminal.