Fargo's Mod/Skeletron (Eternity Mode)

Eternity Mode Skeletron is a pre-Hardmode. It is a more powerful version of Skeletron with more attacks and new behavior.

Behavior
Skeletron has two main attack modes that alter its attacks and behavior throughout both of its phases: idle and spinning. It indicates that it's about to spin by spawning a red, spinning reticle on the player. When Skeletron begins a spin, it also spawns diagonal walls of Skeletron Guardians. While spinning, it sprays rings of Bones with increasing speed as its life decreases.

Phase 1
Skeletron spawns with two hands that try to constantly ram the player. Under 50% health, these hands can spawn rings of Bones that are heavily affected by gravity and homing Cursed Skulls. When one of its hands is defeated, it'll start firing several homing Cursed Skulls at the player.

Under 75% health, Skeletron will enter phase 2.

Phase 2
Skeletron keeps all of its attacks from the previous phase and adds an attack for each state. While not spinning, Skeletron will fire wide V spreads of slowly accelerating Baby Guardians. There will always be a large empty space for the player to go through. This space occurs where the player originally was when Skeletron preformed the attack. While spinning, it'll spawn several Baby Guardians behind it, which then quickly home onto the player.

When Skeletron reaches 0 health, it'll enter a brief Dungeon Guardian state. During this, it will regain 50 health and 9999 defense, and it loses all other attacks except the Bone ring spam. It will start spinning as it slowly accelerates until reaching full speed. After this state is over, Skeletron will be defeated.

Trivia

 * If the Eternity Mode Recolors option is turned on in the Mod Configurations menu, it resembles the Lich boss, specifically his second phase, from Enter the Gungeon.