Atomos is an optional boss in the Dawn of Souls and succeeding versions of the original Final Fantasy. It can be fought on Floor B10 of the Lifespring Grotto, and must be defeated to progess further into the dungeon. Like the other bosses in the Lifespring Grotto, Atomos originally appeared as a boss in Final Fantasy V.
Atomos's signature attack, Wormhole, can instantly kill a single character. NulDeath, Lunar Curtains or Protect Rings are suggested. Firaga and Comet are Atomos's biggest threats beyond Wormhole, but NulBlaze or Fire-resistant equipment like the Ice Armor works well against Firaga. Neither Firaga or Comet should be underestimated, though. Atomos has one of the highest Intelligence stats in the entire game and his Firaga can deal 300 HP of damage to the entire group, with Comet dealing around the same amount of non-elemental damage to a single target.
If the party doesn't have any protection against instant death, Atomos should be defeated as quickly as possible to avoid Wormhole hitting too many times. As with most bosses in the game, casting Saber (with the Giant's Gloves), Haste and Temper on physical units helps to quickly defeat Atomos. If the party is at a high enough level, Wormhole will almost always miss, making the battle much easier.