In The Legend of Zelda: A Link to the Past, the dark realm is called the Dark World.
The Dark World is a parallel version of Hyrule created when Ganon obtained the Triforce and transformed the Sacred Realm through his evil desires. Although its geography broadly mirrors the Light World, its landscapes, inhabitants, enemies, and dungeons are dramatically altered. Familiar places often have darker or more dangerous counterparts.
Link first begins the game in the Light World, where he investigates Agahnim’s plot and obtains the Master Sword. After confronting Agahnim, Link is sent into the Dark World and must rescue the descendants of the Seven Sages from its dungeons. Travel between the realms becomes a major part of the game’s puzzles and exploration.
The Dark World is sometimes confused with the Sacred Realm because it originated from that realm after Ganon’s corruption. It is also distinct from Termina and the Twilight Realm, which belong to other Zelda games and have separate histories. The game’s two-world structure became one of its defining innovations.