Ultranium/Ultrum

Ultrum is a Post-Moon Lord boss intended to be fought after Moon Lord alongside.

While Ultrum is alive, the Guardians phase 1 music will play in phase 1 of the fight and Guardians phase 2 will play in phase 2 of the fight.

Summoning
Ultrum does not spawn on its own and is summoned by using an on the Surface. If brought Underground, it will despawn.

Phase 1
Ultrum slowly chases after the player while firing spreads of green bolts that fly a certain distance before redirecting themselves at the player. Every time he fires 5 spreads, he will stop in place and use one of several alternative attacks in a cycle.
 * Fires two curving rings of green blasts.
 * Fires three massive green tornadoes.
 * Charges at the player twice while leaving behind a trail of green orbs that explode into more shots after a short delay.
 * Fires 4 spreads of green orbs that slow down before exploding into more shots.

Below 50% HP, Ultrum briefly goes invulnerable and transitions into Phase 2.

Phase 2
Ultrum starts using a newer, enhanced attack pattern.
 * Charges eight times.
 * Stops and fires 4 rings of small green bullets.
 * Chases while firing eight wider spreads of redirecting bolts.
 * Stops and five bigger rings of curving green blasts that form a ring around him, before summoning two rows of green fireballs from above and below, telegraphed by green lines.

Phase 3
At roughly 12.5% HP, Ultrum briefly goes invulnerable, stops moving normally, and releases a desperation attack.
 * Fires two fast rings of green shards.
 * Fires a long stream of green shards at the player.
 * Summons three rings of green shards around the player that converge on them.
 * Teleports above the player and repeats.