Orthax

Orthax is a shadow demon who makes vengeance pacts. As an NPC, it is played by Matthew Mercer.

It appeared to Percy, in what he thought was a dream, and offered him vengeance against those who destroyed his family. While on the hunt for the Briarwoods, Percy began to act increasingly more erratic and violent due to Orthax's influence. During several fights, Orthax shrouded Percy in smoke.

When Percy tried to sever his ties to Orthax, it appeared and fought Vox Machina. As a precaution, Scanlan threw Percy's pepperbox pistol into acid, in hopes that that would sever its tie to him.

Orthax reappeared when Vox Machina caught up to Anna Ripley, as she had made a similar pact with it, and assisted her in the fight against Vox Machina on the Isle of Glintshore, which led to Percy's death at the hands of Ripley.

Vox Machina was able to fell the demon, but it remained in the Material Plane through its attachment to Ripley's pepperbox, Animus. It used this attachment to grab hold of Percy's soul, preventing his resurrection until Keyleth managed to sever the thread connecting it to Animus through the use of Greater Restoration.