Karil the Tainted is encountered in the Heist minigame. He appears in the Canifis scenario. Players on the Robbers team can take his form to disguise themselves and evade players on the Guards team.
The only way to obtain this disguise is to dig with spades on a mound on the Canifis map. This may cause a random Barrows brother (one of the six) to appear nearby for a short time. Note that not every Canifis map will contain a Barrows mound, and usually a player doesn't manage to uncover anything when digging in the mound.
This information has been compiled as part of the update history project. Some updates may not be included - see here for how to help out!