The Demon Ghosts are a quartet of spectral beings who pass by Leonardo and Ancient One's campsite in the episode "The Ancient One". Leonardo attacks them, even though they were just drifting through. Since he can't make contact with them, but they can with him, he assumes that he cannot win the battle. However, Ancient One convinces Leonardo to surrender, and the Demon Ghosts stop attacking, as the ancient one revealed another lesson; "A Warrior who never fails, never learns" and Leonardo and Ancient One returns to their journey.
The Demon Ghosts are voiced by Sean Schemmel and Marc Thompson.