The Atom Splitter is the Legendary Monsters gun, an Annihilation cannon built from 
Portal Shards after you beat The Obliterator. Hold right-click for about a second and it fires a beam that does 20 through armor to everything in a 30 block line, or a cluster bomb if you’re sneaking. It never breaks, and it reloads itself from Bottles Of Annihilation in your inventory.
It’s the best ranged damage in the mod by a distance, and the ammo is an 
Eye of Ender and a 
Glass Bottle for four reloads’ worth. The only real cost is the two Portal Shards.
Obtaining
It’s crafted from 2 Bottles Of Annihilation, 2 Portal Shards, an 
End Rod and an 
Enderitium Ingot, with the bottles on top, the rod and shards in the middle row and the ingot at the bottom.
The Portal Shards are the gate, as they only drop from The Obliterator, 2 to 4 per kill, so one boss fight covers the gun with a shard or two spare.
8Enderitium Ingots come from the 
Enderitium Gems the Annihilation Pursuers drop and the Space Station Chests, and an End Rod is any End City or a 
Blaze Rod over 
Popped Chorus Fruit. Bottles Of Annihilation are an Eye of Ender over a Glass Bottle, eight at a time, and you’ll want a stack of them for the ammo anyway.
The same shards build 
The Tesseract and the Annihilator’s Armor Set, so a second Obliterator kill is usually in your future. The gun is the one to build first if the fights you lose are the ones where things won’t come to you.
Usage
Hold right-click and it charges like a 
Bow, and once it’s been held for about a second, releasing fires. There’s a 1.5 second cooldown after each shot, and both fire modes spend one of its 3 shots and deal Annihilation damage, which ignores armor points and stacks the Annihilation effect on whatever it hits.
Annihilation Beam
Release without sneaking and it fires a straight beam 30 blocks long that hits everything in the line for 20, sets it on fire for 2 seconds and stacks Annihilation. Fire it level with the ground, within about 20 degrees of flat, and it also walks a chain of explosions along the floor for 8 each, so flat arenas are where it does its best work.
Cluster Bomb
Sneak while you charge and it lobs a Cluster Annihilation Bomb instead, which does 10 on a direct hit and then bursts into two dozen small bombs that scatter around the impact. Each of those does 11 plus 3% of the target’s maximum health, so a pack standing on the landing spot takes far more than the beam would give them.
The beam is for lines and single tough targets, the bomb is for crowds and for anything that just took cover behind a doorway. Neither fires without the charge, so start holding before the fight starts rather than when something is already on you.
Both modes go through a raised 
Shield‘s damage reduction the same way, because Annihilation damage isn’t stopped by armor points, only by Protection. The Annihilation effect it stacks slows the target’s attacks, and the Status Effects page covers the levels.
Ammunition
The durability bar is the ammo gauge, and it’s 3 shots from full. When it’s empty a right-click reloads it on the spot, taking 2 Bottles Of Annihilation out of your inventory, and it can’t be reloaded early, so the count on the bar is the count you have.
One Eye of Ender and one Glass Bottle craft 8 Bottles Of Annihilation, so a stack of 
Ender Pearls and 
Blaze Powder is a very long time at the trigger. The gun itself never takes real damage, and the Space Station‘s Chests hold bottles too.
Enchantments
It takes Unbreaking and Mending from an 
Anvil because the game reads the ammo bar as durability, and both are wasted, since the bar only moves when you fire and refills from bottles. No damage enchantment applies to it, so leave the levels for your sword and your armor.
Tips
Charge while you walk backward and let the target come into the line, since a beam through three Flameborn is three hits for one shot. Keep the ground flat between you and the target if you want the explosion trail, because a beam aimed up or down at more than 20 degrees skips it.
Carry the bottles in your inventory and the gun reloads without a menu, which matters more than it sounds mid-fight. Against The Obliterator the beam’s 20 is exactly his damage cap, so every shot is a full hit, and the sneak bomb is the answer to the clones he spawns in his second phase.
FAQ
Craft it from 2 Bottles Of Annihilation, 2 Portal Shards, an End Rod and an Enderitium Ingot at a 
Crafting Table. The Portal Shards only drop from The Obliterator in the Space Station, 2 to 4 per kill, so you have to beat that boss first.
Right-click it while it’s empty with at least 2 Bottles Of Annihilation somewhere in your inventory, and it reloads to a full 3 shots on the spot. The bottles are an Eye of Ender over a Glass Bottle, eight per craft, and the Space Station Chests carry them as well.
The beam does 20 to everything in a 30 block line, ignoring armor points, plus 2 seconds of fire and a trail of 8 damage explosions when it’s fired level. The cluster bomb does 10 on a direct hit and scatters two dozen small bombs that each do 11 plus 3% of the target’s maximum health.
No, the bar on it is an ammo counter rather than durability, and the gun never takes real damage. When the bar runs out you reload with 2 Bottles Of Annihilation, and Unbreaking or Mending do nothing for it.
