Abyssion, The Forgotten One is a Hardmode, post-Plantera boss fought in the Aquatic Depths. The boss has multiple forms, each more powerful than the last.
The Forgotten One does not spawn on its own and requires the player to kill Aquatic Hallucinations in the Aquatic Depths. They will drop Abyssal Shadows, which will inflict the False Presence debuff once for every one picked up. Upon reaching three stacks of the debuff, The Forgotten One will emerge. Multiple Abyssions cannot be summoned through this process.
- While below 90% life, Abyssion will occasionally hide within his shell, greatly increasing his defense. When in his shell, he will rapidly barrage the nearest player with fast moving and heavy damaging spit projectiles.
- While below 65% life, Abyssion will release large spinning whirlpools that deal massive damage to the player and knock them back, even if the hit player has an effect similar to the Cobalt Shield.
- While below 55% life, Abyssion will start spawning five Abyssal Spawn every time he hides within his shell.
- While below 50% life, Abyssion will release faded water ripples. These bounce along all walls, and stay within the area for 4 seconds.
- Upon reaching 0 health, it transforms into its second Cracked stage. The transformation is notified with the status message, "Abyssion's shell violently cracks!"
- While in this state, Abyssion will keep all of his previous attacks without the need of lost life. They will be quicker, deal more damage, and come out far more rapidly too.
- Additionally, Abyssion will start releasing semi-homing slashes of dark magic, that attempt to blockade the players movements.
- If taken outside of the Aquatic Depths, he will enrage in a manner similar to Plantera.
Abyssion, the Forgotten One (Released) is the Expert Mode exclusive final phase of the encounter. The transformation is notified with the status message, "Abyssion breaks free of its ancient prison!"
- While in this state, Abyssion will keep all of his previous attacks from his two previous forms without the need of lost life.
- Additionally, Abyssion will move 50% faster and each attack will be infused with dark magic, dealing even more damage.
- Upon defeating Abyssion for the first time, the Aquatic Depths biome's Choking Aura debuff will be replaced with the Untainted Waters buff, allowing them to breathe water and air in the biome without taking damage. This is indicated with the message upon its defeat "The toxic haze of the depths has been lifted..."