Soul of Chaos (Final Fantasy)

Soul of Chaos is the collective name of the four bonus dungeons originating in the Dawn of Souls version of Final Fantasy, and reappearing in the 20th Anniversary Edition:

(in order of unsealing)
 * Earthgift Shrine
 * Hellfire Chasm
 * Lifespring Grotto
 * Whisperwind Cove

Each dungeon is tied to one of the four worldly elements (Earth, Fire, Water and Wind respectively), and they are initially sealed by a statue of their element's respective Chaos Fiend (Lich for Earth, Marilith for Fire, Kraken for Water and Tiamat for Wind). Once a Chaos Fiend is defeated and the elemental Crystal is restored, their statue will disappear, opening the way to the dungeon.

Each dungeon, with the exception of Earthgift Shrine, is significantly longer than any in the main game: Earthgift Shrine has 5 floors, Hellfire Chasm has 10, Lifespring Grotto has 20 and Whisperwind Cove has 40 floors. Every tenth floor has an exit portal to the World Map and a number of bosses available to battle, all major bosses from future games in the series: Earthgift Shrine features the Dark Crystal guardians from Final Fantasy III, Hellfire Chasm features the Elemental Lords from Final Fantasy IV, Lifespring Grotto features bosses encountered in the Interdimensional Rift (except Gilgamesh and Atomos) from Final Fantasy V and Whisperwind Cove features four of the more notorious bosses from Final Fantasy VI.