A Hyudoro, being a ghost, lacks any form of legs and has the ability to hover and fly, along with being able to move through walls and floors. It is a white ghastly form with two stubby arms, a pair of angry black eyes, and two horns that protrude from its head.
Sonic & Knuckles
In Sonic & Knuckles, the player unavoidably encounters a pack of Hyudoros in Act 2 of Sandopolis Zone. It is inferred that the enemies were forced into a capsule by Doctor Robotnik long before Sonic arrived at the zone. Once Sonic/Tails opens the capsule, the Hyudors will fly out and vanish from sight. As the light in the zone progressively gets darker, however, the Hyudoros will reappear and strafe the character at the top of the screen, where they cannot damage the player nor be defeated.
Hyudoros appear in three different forms, depending on how dark the act currently is. The darker the act becomes, the stronger the Hyudoros will become:
- FULL LIGHT: In full light, no Hyudoros are visible.
- HALF LIGHT: In half light, a single Hyudoro appears at the top of the screen. The Hyudoro is small in size and has docile eyes, along with lacking any arms or horns.
- PARTIAL LIGHT: In partial light, the Hyudoro will grow slightly bigger, gaining arms and a more menacing expression. A second Hyudoro also becomes visible and joins the first.
- FULL DARKNESS: In full darkness, the Hyudoros, now a group of three, will grow even bigger. Horns will appear on their head, and their eyes will furrow into an even angrier expression. In this state, the ghosts also gain the ability to attack the player, relentlessly spawning and charging at the character.
In order to keep Hyudoros from attacking, the player can pull on the many light switches scattered around the level; doing so will reset the light level to full light and cause any Hyudoros to despawn for about 15 seconds.
Occasionally, if the player is moving fast enough during the attack phase, the Hyudoros may fall off-screen, but they will eventually respawn. Before a Hyudoro starts attacking, they are locked at the top of the screen at all times. Just before a Hyudoro attacks, it will pause briefly before moving slowly in the direction of the player.
As Knuckles the Echidna, the ghosts are already free, since Sonic opened the capsule before Knuckles reached Sandopolis Zone. This is especially dangerous at the beginning of Act 2, since the act starts off completely dark and Knuckles has no Rings.
Once the player reaches the boss area, the Hyudoros will disappear completely and the lights will turn on automatically and permanently.
Sonic Rivals 2
In Sonic Rivals 2, the Hyudoros appear as enemies in Mystic Haunt Zone. In this game, only one Hyudoro appears during one mansion section of the level. The player cannot defeat the Hyudoro, but it is possible to make it disappear by pressing down the switches to light up the interior mansion before the Hyudoro attacks.
In other media
Sonic the Comic
In the Sonic the Comic series published by Fleetway, the Hyudoros are ghosts inhabiting the Lost Pyramid of Sandopolis. As a part of the pyramid's curse, they will attack any intruders when the light starts to fade. During the comic's run, they attacked Captain Plunder and his crew when they tried to steal the pyramid's treasure and the Chaotix Crew when they paid a visit.
- 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 the 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 the Hyudoros disappear, another one comes to take place.
- In Sonic & Knuckles, even though players can avoid opening the capsule containing Hyudoros with Debug Mode or glitching, they will still appear if the player touches any Checkpoint.
- 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.
- Three Hyudoro make an appearance in the Sandopolis Zone sub-area of the Sonic the Hedgehog World in Lego Dimensions. As in Sonic & Knuckles, they appear after opening a lone capsule. Upon being freed, they split a Gold Brick into three pieces and hide in dark corners of the map. The player must use the Shift Keystone to get near their hiding spots, then push a switch to open the doors they hide behind and defeat them using a character with the Illumination ability.
- The Hyudoros' name is a Japanese onomatopoeia for the sound a ghost would make.
- Sonic The Comic Issue #43 (Sonic & Knuckles Special Part 1)
- "Here's the desert stage, something that hadn't appeared in the series until this point, so we were able to make it relatively free of earlier influences. The ghosts appearing in the second half are the first enemies who weren't made by Eggman. When he infiltrated the pyramid, he must have worked very hard to trap them inside that box." - Takashi Iizuka, Sonic Team Commentary in Sonic Jam Strategy Guide.