Astrum Deus

"A star-spawned horror tunnels through the astral infection."

Astrum Deus is a Hardmode boss that must be defeated in order to allow Astral Ore to be mined. It begins as a large worm, similar to The Destroyer, but splits later in the fight.

Spawn
Astrum Deus does not spawn on its own and can be summoned by using the Astral Beacon with a Titan Heart or Starcore in the inventory at night. Astrum Deus can be spawned even in early Hardmode with Titan Hearts, but the boss is intended to be fought after the Lunatic Cultist has been defeated. When successfully activated, two astral stars rise from the beacon and eventually combine to summon Astrum Deus.

Behavior
Astrum Deus behaves like a regular worm enemy, moving through blocks and attempting to ram the player, but it has the ability to fly as well. While flying, it periodically fires lasers from its body.

When its health is reduced to 50%, it becomes invincible and shortly splits into two shorter worms. They both continue using their previous attacks while attempting to ram the player. Both of the worms take turns exclusively shooting lasers and releasing mines, which is represented by a subtle color shift. The lasers shot by both worms are fired at a slower succession. If one of these worms are killed, the other will despawn and the fight will end. Astrum Deus has segments in total. When split, both worms have segments in total.

If the player is far away from Astrum Deus, it will accelerate much faster and have a higher top speed until it is back within their range. If gravity is reversed, such as when using a Gravitation Potion, the worm gains a significant speed boost and fires more lasers and mines. If Astrum Deus is alive when it becomes daytime, it will enrage, moving faster and shooting lasers and mines at a higher rate.

Attacks

 * Will attempt to ram the player often.
 * Will occasionally fire a barrage of lasers that follow one of three patterns:
 * A stream starting at the segment closest to its head, which ends at its tail.
 * A stream starting at the segment closest to its head, which ends at its middle segment. This pattern is performed slower than the others.
 * Two streams starting at the segment closest to its head and its middle segment, which ends at its middle segment and tail respectively. This pattern is performed the most frequently as Phase 1 progresses.
 * Upon reaching 80% health, it can also leave mines as it attacks, which explode after lingering for a while. Occasionally, it stops firing lasers and releases mines more frequently.

In Revengeance Mode

 * Attacks more frequently in Phase 2.
 * Moves faster.
 * Inflicts Extreme Gravity.

In Death Mode

 * The worm and its lasers are faster, and they gain velocity as their health decreases.
 * Can release astral mines at all times, and fires more of them at a higher frequency.

Aftermath

 * Once defeated, Astral Ore will become mineable.
 * Astral Crates will contain Astral Ore, Astral Bars and Meld Blobs.
 * Using Novae Slag or Celestial Remains in the Extractinator will have a chance of giving the player Astral Ore.

Resistances

 * All Astrum Deus segments take reduced damage from non-minion piercing projectiles in Expert Mode. Projectile damage is divided by the number of times it pierces. Infinite piercing deals 50% damage.
 * All segments reduce damage from grenade-type projectiles by 80%. A list of affected projectiles is below.


 * All segments also take:
 * 20% damage from the explosions and rainbow bolts from the Grand Guardian.
 * 25% damage from Plaguenades and Stardust Dragon.
 * 50% damage from Aerial Bane's arrows, Forbidden Sun, and Storm Ruler's tornadoes.
 * 60% damage from Electrospheres which are fired from Electrosphere Launcher, Pulse Dragon, and Sky Dragon's Fury.
 * 75% damage from Charged Blaster Cannon's laser, Eidolon Staff's ice cluster, Inferno Fork, Lazhar, Plague Staff, and Resurrection Butterfly.

Tips

 * Try and keep moving, and don't stay in one spot. Avoid circling the boss as the astral orbs will make it too difficult to do so.
 * Be aware of the position of the mines as they can wall off the player, making it extremely difficult to break out and leaves the player as easy prey.
 * During Phase 2, the trail of mines will be shorter, although there will be more trails left throughout the arena.
 * Having a large arena can make this much less of an issue.
 * Astrum Deus’ abundance of projectiles can, especially on less powerful computers, cause large frame rate drops. This can be used to the player's advantage by turning frame skip off, giving them more time to process and dodge attacks.
 * Like several other bosses, Astrum Deus will not despawn no matter the distance the player travels from it. This means the Magic Mirror can be used to teleport to a more suitable spot to fight.

Trivia

 * Astrum Deus' theme is Pest of the Cosmos, which was composed by the artist DM DOKURO.
 * If the Calamity Music add-on mod is disabled, Boss 3 will play instead.
 * "Astrum Deus" in Latin roughly translates to "The Star God".
 * Astrum Deus used to be spawned by killing three Atlases after the Lunatic Cultist was defeated.
 * In previous versions of the mod, the fight consisted of one larger worm with ten smaller worms as minions.