Hyudoro

Hyudoros (ヒュードロ) are ghost enemies that made first appearance in Sonic & Knuckles. They are ghosts of beings that were trapped inside the Capsule. Hyudoros are freed by the player at the beginning of Sandopolis Zone Act 2 and start to attack him as the Zone gets completely dark.

Appearance
Hyudoros are ghosts with a horrific appearance usually existing in dark and ghastly places. In Sonic & Knuckles, they are presumably ghosts of starving animal friends inside the single Capsule. As they start appear they are shown to be small ghosts, but as the lights start turn off, they grow two horns on their head and have black void eyes.

Sonic & Knuckles
In Sonic & Knuckles, the player encounters the Hyudoros in Sandopolis Zone Act 2, after the player opens a capsule. Hyudoros cannot be destroyed; if the player jumps, they follow him. If the player goes too fast, they disappear and meet him wherever the player is. They don't try to hurt the player until it is completely dark. The player also can't touch them as they fly above until they are in their perfect form. These ghosts appear in three different forms, depending on how dark it is. The darker the act becomes, the stronger the Hyudoros will be:


 * 1) In full light, there are no Hyudoros.
 * 2) In semi-brightness, one small Hyudoro appears to be floating above.
 * 3) In semi-darkness, two evil-looking Hyudoros appear hovering, while they don't start attacking.
 * 4) In complete darkness, three Hyudoros with horns on their heads appear and start to attack the player.

The only way to make the Hyudoros disappear for a while is to pull down the light switches located around the level. The Hyudoros disappear completely and the lights go on when the player falls into the arena where the boss of zone is encountered. The player Spin Jumps on them during the attack, but reappear right after a second and continue their assault. If the player is invincible then a similar effect will occur. As Knuckles the Echidna, the ghosts seem to already be free, since Sonic opened the capsule earlier. This is especially dangerous at the beginning of Act 2, since the act starts off completely dark and Knuckles has no Rings.

Sonic Rivals 2
After many years, Hyudoros reappear in Sonic Rivals 2. They appear as enemies at the Mystic Haunt, but this time only one Hyudoro appears, during one mansion section of the level. The player might not be able to defeat the Hyudoro himself but it is possible to make it disappear by pressing down the button before Hyudoro attacks.

The boss of the Mystic Haunt appears to be big variation of Hyudoro with horns, piloted by Eggman Nega. It is referred to as the "Egg Phantom". Like normal Hyudoros, its weakness is revealed to be the light.

Sonic the Comic
In Fleetway's Sonic the Comic, Hyudoros are mentioned as Sandopolis Ghosts, and appear only one time, when Captain Plunder, Filch and the Professor attempt to leave the pyramid with several bags full of its treasure. Sonic tries to chase them, but Knuckles stops him, knowing the hazards of pyramids. As the place gets dark, small ghosts starts to follow Plunder and the others. Filch sees the ghosts and tries to warn Plunder and Per'fessor, but they don't see them. After that, three ghosts appear in front of them. Filch believes them to be "the wrong sort of ghosts", and the leering phantoms chase the pirates out of the pyramid, which seals shut behind them.

The Sandopolis Ghost has a slightly different appearance compared to its game counterpart, as it normally appears to be small blob, but it eventually develops a more sinister look and a full body of cloaked phantasms with arms.

Trivia

 * As the Sonic Jam strategy guide states, Hyudoros weren't created by Dr. Eggman. It also reveals that Eggman infiltrated into the pyramid and trapped the ghosts inside the Capsule, working very hard.
 * Several media mentions that Sandopolis Zone Act 2 takes place in a cursed pyramid, with Dr. Robotnik trying to trap Sonic inside.
 * When the capsule is opened inside of pyramid, it releases six Hyudoros even though only three of them start to follow the player. It is unknown where the three others have gone, although it is possible that as one of Hyudoros disappear, another one comes to take place.
 * Hyudoros and Boos are considered to be separate ghost species, but there are a few similarities. Both of them have resided in a dark pyramid and are able to disappear and come back.