The Ptolemaic Army seek to release this demon from the dimension where it is imprisoned. The cult leaders of the Ptolemaics are controlled by it through their ritual masks, making it the true mastermind of the game's events despite its existence not being known until the game's final battle. Its origins and purpose are a complete mystery.
The Avatar of Evil appears as a gigantic gargoyle-like demon with bat wings carrying a huge scythe. Its body is pitch black, with supernatural patterns on its body that resemble a night sky. Its face is covered by darkness, with only its glowing red eyes and a few strands of what appears to be hair visible. It also has a skull face on it's chest, which acts as the demon's weak point. The Evil Spirit is much larger than even the Invader King, currently standing in as the largest boss in the series.
Its approaching is indicated by a thunder storm in the background, after which the sky goes black and it's silhouette is seen on the distance. It attacks by shooting energy blasts aimed at the player that crash down to the ground, after which it flies away and raises its scythe. As the scythe hits the ground, it scatters smoke clouds which must be avoided.
Upon being defeated, the Avatar of Evil screams and flies away, after which the sky returns to normal, signaling that it has returned to wherever it came from.
- Prior to its debut as a playable unit in Metal Slug Attack, the Avatar of Evil was known more commonly by Metal Slug fans as "Scyther" or "Evil Spirit Incarnate".
- It is the largest boss to have appeared in the series.
- The Avatar of Evil resembles the alternate flag of not only the Ptomelaic Army, but also of the Rebel Army.
- It also bears a passing resemblance to The Grim Reaper, given the occult nature of the Ptolemaic Army's doings, some fans believe the Avatar of Evil to be Death itself, given form and summoned to do the Ptolemaic's bidding. Others think that it is a demon of some kind, or possibly even the Devil. It could also simply be an extraterrestrial life form, given the starry night appearance of its "skin".
- At the beginning of the fight, the players' characters use their cold/chill animation, suggesting that the boss may have a cold body or the atmosphere that the fight takes place in is very high where a person may get cold.