Supernatural entities

There are numerous undead, phantasmal, fantastical, or otherwise supernatural entities that inhabit the Sonic universe. More often then not these beings inhabit areas that have a reputation for being haunted.

Game Appearances

Sonic and Knuckles

Inside of Sandopolis Zone's Act 2 pyramid are several ghosts who are trapped in a capsule. After Sonic or Tails unwittingly frees them, they begin to hoover about the top of the screen, becoming more menacing if the stage is allowed to become darker. Some time after the level becomes completely dark, they begin to charge the player and can inflict damage. Although they can be beaten with a spin attack or spin jump, they will reappear shortly afterward. Knuckles has to deal with these belligerent spirits from the get-go.

Sonic Adventure

Chaos seems to fit the bill as an supernatural entity, given his abilities to shape shift, split and reform, and summon twisters in his Perfect form. If he has any similarity to Chaos Chao, then we may assume that he is immortal.

The creepy prisoners of the interior of Red Mountain don't seem to be quite human. At a reasonably far distance, one can see that they're composed of disembodied strips and yellow eyes. They mostly function as scenery, although in Mission 30 of Sonic Adventure DX, one has apparently escaped from its cell and the player must track him down.

Finally, in the Halloween Party download available for the original Sonic Adventure from October 31st, 1999 to December 31st, pumpkin-headed ghosts donning hats, cloaks and lanterns haunt Twinkle Park. Although they don't hurt the player, they do tend to hoover around in inconvenient places. Regardless, they'll greet the player with a friendly "Trick or Treat!" when approached.

Sonic Adventure 2

Ghosts reappear as proper enemies, haunting desolate mountainous regions and Egyptian ruins. Tiny Boos either wander about or hide in scenery, and spook the player when approached. Normal sized Boos either home in on the player and restrain them temporarily, or bump into the player and cause damage. Larger Boom Boos cause damage on contact and take three attacks to vanquish.

The gigantic King Boom Boo acts as a boss for Knuckles. Unlike other ghosts, who appear at worst only slightly hostile, King Boom Boo attempts to off Knuckles by chomping him and burning him with projectiles and streams of ghostly blue fire, cackling and moaning all the while. Luckily, King Boom Boo is damageable when exposed to sunlight. Because Knuckles has to dally with this phantom monarch, Eggman manages to confront the intruders of his latest base with the equally formidable Egg Golem before they can hijack his space shuttle.

Sonic Heroes

Pumpkin-headed ghosts of several sizes wander the abbeys and halls of Hang Castle and Mystic Mansion. Although they do cause rings to slip from your grasp when touched, they don't generally go out of their way to hurt you, preferring to mill about a small area. A slightly more antagonistic variety tends to suddenly appear as your team approaches. Either type will disappear if hit with any attack, though they'll reappear shortly. At least one group functions as a Homing Attack chain, allowing you to clear a wide gap.

More overlooked is a harmless skeleton who spies on your team in Mystic Mansion. The Peeping Tom always manages to slip behind a wall and disappear before you can ever get to him.

Shadow the Hedgehog

Depressed-looking blue ghosts manage to infiltrate the nooks and crannies of Eggman's Cryptic Castle. They slowly rise from vents and towers across the property and cause damage on contact.

The dank underground of Cryptic Castle also contains a skeletal monstrosity who pursues Shadow as he rails towards the Goal Ring. It's not known if this creature is mechanical, supernatural, or otherwise.

Black Doom can create temporary transparent replicas of himself. Although these false images aren't the real deal, they have no problem summoning damaging attacks.