Vitreous

Vitreous is the boss of Misery Mire and captor of the sixth Maiden in. It is a gigantic eyeball surrounded by smaller eyeballs, immersed in an acidic green liquid.

Strategy
The main eyeball spends most of the battle immersed in the slime, safe from attack. If Link attempts to enter the slime, he will be hurt. Occassionally Vitreous will shoot lightning at Link from its huge iris, similar to one of Agahnim's attacks, while the smaller eyeballs will continually attempt to swarm him, attacking one by one. To avoid this, the best method is to stand in one of the corners of the room and use the Whirling Blade Attack or repeated sword strikes to hurt the eyes as they close in. As each eyeball is eventually destroyed, the intervals between Vitreous' blasts of electricity will shorten. Once the smaller eyeballs have been taken out, the main eyeball will attempt to charge at Link. This giant, massive eyeball can be defeated with sword strikes, but can be kept at a distance using the Bow, which also causes more damage. Defeating Vitreous earns Link a Heart Container and the sixth imprisoned Maiden.

Alternatively, Link can also use the Magic Cape or the Cane of Byrna to dash right into Vitreous and attack him and the smaller eyesballs until it is defeated. The protective ability of these weapons allows Link to get through the dangerous slime and attack the mass of eyeballs while they are inside.

A Link to the Past comic
Vitreous makes an appearance in Chapter 9 of the A Link to the Past comic. After Link falls for the Wizzrobe's trap by heading into Misery Mire, the Wizzrobe conjures Vitreous from the swamp to devour him. However, as Vitreous approaches Link, a whirlpool is created that engulfs both Vitreous and the Wizzrobe, and from beneath Zora appears to rescue the young hero.

A Link to the Past manga
Vitreous appears in a very different form in the manga. Vitreous appears as a girl who was transformed into a cyclops monster by the Dark World due to her spying on others. She is friendly, and helps Link and Ghanti by telling them where Zelda is hidden.

Trivia

 * Zol drop from the ceiling of a room below Vitreous, similar to how they drop from the ceiling before fighting Slime Eye, a similar boss.
 * The name "Vitreous" comes from Vitreous humor, the clear gel found inside the eyeballs of vertebrates.
 * Vitreous is called "Gerudōga" (Geldogger is a rough translation) in Japanese. The suffix -dogger, in addition to its eyeball appearance, connects it to Digdogger, a recurring boss in the Zelda series. The prefix gel-, pronounced with a hard G sound, is likely a reference to the Gel enemies, whose name is pronounced with the same sound in Japanese rather than like the English word "gel." The beginning of the full name, possibly coincidentally, also sounds like a contraction of the geld- prefixes of the desert-dwelling enemies Geldarm and Geldman; although Vitreous is found in a swamp, its location in the Dark World corresponds to the Light World's Desert of Mystery, which contains the Geldman enemies.
 * In the deeper part of the liquid, there are four eyes who never attack. They are destroyed only when Vitreous is defeated.