Archeon/The Hallowed One

The Hallowed One is a Hardmode worm-type boss.

The Hallowed One is supposed to be fought right after Plantera, since s are needed in order to craft the summon item.

Summoning
The Hallowed One cannot spawn naturally and can only be summoned by using in The Hallow at Night Time.

1st Phase
The Hallowed One burrows through the ground, spawning orbs of Hallowed particles which then split into 2 projectiles. The first projectile flies directly upwards, the second projectile flies directly downwards and is a bit faster than the other projectile. It will also shoot one (4 in Expert Mode) homing projectile(s) out of its head, which trail the player for a long time.

2nd Phase
At around 25-30% health, The Hallowed One becomes nearly invincible and summons 2 Hallowed Twins to fight for it. When the Hallowed Twins are alive, The Hallowed One will also become slower and less aggressive, allowing you to focus on dodging the minions.

The Hallowed Twins are like The Hallowed One, but they are less powerful and a bit faster.

Aftermath
A will fall from the sky, bringing new enemies and items into the player's world.

Lore
A regular worm that feasted on thousands of Prismite fish and hallowed creatures to grow impossibly massive and destructive. All the heroes who tried stopping It were eaten immediately, since everyone underestimated It and Its power...