Abyssion, The Forgotten One

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.

While The Forgotten One is alive, the vanilla track Boss 3 will play.

Spawn
Abyssion 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, Abyssion will emerge. Multiple Abyssions cannot be summoned through this process.

After Abyssion has been defeated in the world at least once, Abyssal Shadows will behave like normal Summoning items, summoning the boss on use.

Attacks

 * If the player is not in a body of water, it will enrage in a manner similar to Plantera. While enraged, it moves at a higher velocity, summons Abyssal Spawns more often, and its spit projectiles will home in on the player.
 * Abyssion slowly follows the player, attempting to swim into them. It will also periodically fire wall-piercing whirlpool projectiles.
 * Immediately at 85% life and occasionally below it, Abyssion will occasionally hide within its shell, causing it to take 1 damage per hit. When in its shell, it will rapidly shoot at the nearest player with fast moving and heavy damaging spit projectiles.
 * At 50%/ life, Abyssion's shell cracks as it enters its second phase, immediately launching its shell attack and gaining numerous changes.
 * Starts periodically shooting pairs of projectiles together; water ripples that bounce along walls, and semi-homing dark magic slashes that attempt to block player movement.
 * Abyssion's shell attack can be used much more frequently, but only reduces damage by roughly 50%. It can also use all of its other regular attacks while in its shell, as well as summoning 5 Abyssal Spawn whenever it enters this attack.
 * At 33% life in Expert Mode, Abyssion transforms into its third phase.

In Expert Mode
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!"

Behavior

 * Movement speed increased by roughly 50%.
 * Abyssion immediately uses its spit attack after entering this phase.
 * Abyssion retains all previous attacks, but with a few changes:
 * Spit attack fires more shots and purple shots that deal increased damage, but Abyssion no longer reduces incoming damage while using the attack.
 * Fires three dark magic slashes at once, which are also purple and inflict much higher damage.

Aftermath

 * 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..."
 * The Diverman will start selling The Whirlpool.

Trivia

 * Abyssion resembles a large Parapaguridae, an aquatic crab that lives in deep water.