|AI Type||Brain of Cthulhu AI|
300051006487 (with creepers) 
|Entering second phase|
- Internal NPC ID: 266
|AI Type||Brain of Cthulhu AI|
- Internal NPC ID: 266
The Brain of Cthulhu can be spawned by fulfilling one of these conditions:
It must then be defeated within the Crimson: getting out of it will make it retreat and despawn.
Brain of Cthulhu has 2 phases, with the latter starting after all Creepers have been dealt with.
The Brain cannot be attacked in this phase, only the Creepers can. The Brain will teleport to a spot in the arena, then slowly advance towards you. The Creepers will follow shortly after, and will clump up with the brain for a few seconds, before the Brain teleports again and the Creepers will follow. The idea is that the Brain will teleport behind you, which will make the Creepers fly into you, causing damage. Jump out of the way, either by using your platforms or with an extra jump, then attack the Creepers. Use piercing weapons to speed up the process. When a Creeper dies it can drop either Crimtane Ore or Tissue Sample.
When all of the Creepers are dead, the Brain will split, revealing a Heart with an Eye on the inside. The Brain teleports much more frequently, often at unexpected angles, and will try to ram into you. The Brain only has 1000 / 1700 / 2167 health, so it can be killed fairly quickly. Keep using your platforms to jump out of the way. Use your strongest weapon to take down the Brain. If you have a weapon with high knockback, it will excel at dealing with the Brain, so much so that it's possible to stand in one location and attack it whenever it gets close.
The Creepers are much more aggressive. They will actively try to fly into you, even ignoring the Brain if it means an opportunity to land an extra hit or two. They can also inflict a large variety of debuffs:
- Confused: reverses the movement buttons;
- Poisoned: saps 2 health per second;
- Bleeding: stops health regeneration;
- Darkness: reduces vision;
- Silenced: makes you unable to cast any Magic weapons;
- Broken Armor: reduces your Defense by half;
- Weak: reduces Melee damage and Attack speed, Movement speed and Defense by 4.
During the Second Phase, the Brain will spawn clones of itself around the player. They serve as decoys, and while they cannot be attacked they can confuse the player. They start off as barely visible but as the Brain loses more health the clones will become more opaque. At low health the clones will be completely visible. The Brain will also become faster as it loses health. Life-draining debuffs such as On Fire and Frostburn, or a Hunter Potion will help distinguish them. Additionally, summoned Minions will always target the correct copy. Unlike the Lunatic Cultist clones, the Brain of Cthulhu clones do not appear on the map, making it trivial to tell which direction the real boss is coming from.
This content is transcluded from Guide:Practical tips § Combat.
- Prepare your Boss-fighting arena with adequate lighting, Campfires and Heart Lanterns for health regeneration, and Stars in Bottles for mana regeneration if you use magic. Bast Statues will provide a significant defensive boost. Garden Gnomes and (if underground) proper torch placement will reduce damage taken and increase damage dealt due to the luck. Note Torch God's Favor helps with this. Honey pools can also be used for extra health regeneration.
- Sunflowers provide the Happy! buff to players within a 50-tile radius, which grants a +10% movement speed bonus and 17% reduction in enemy spawn rate.
- Always carry around one stack of the highest-tier healing potion available (Lesser, Normal, Greater, or Super Healing Potions). Relying only on natural Health Regeneration is not a good idea.
- Bosses are displayed on the Minimap: Follow the boss's icon to track it when you're struggling to find it. You can identify the boss icons by checking their respective wiki pages.
- Once you have freed the Mechanic in the Dungeon, you can use Wire to enhance your arena with Traps and helpful Heart and Star Statues. The former helps to damage the enemies, though be careful to set them up correctly in order not to hurt yourself during the battle. The latter will spawn heart and star pickups, respectively. Connect these devices to timers.
- It may be useful to summon and kill the Eye of Cthulhu or King Slime before summoning any harder boss in order to spawn an extra one-use pool of Hearts. Note that this will prevent any Heart Statues from spawning new Hearts.
- Remember to use the Sharpening Station, Ammo Box, Crystal Ball, Bewitching Table, and Slice of Cake before you go to explore or battle bosses/invasions. These will give boosts that are useful to take, with the first four being class-specific. They give Sharpened, Ammo Box, Clairvoyance, Bewitched, and Sugar Rush, respectively.
Your arena will have to be built in The Crimson, because it can only be summoned there. Since the Brain of Cthulhu spawns as soon as you smash a third Crimson Heart, you're likely to be in a very enclosed area when the fight begins. This can lead to you to being swarmed by the Creepers and get overwhelmed. This can be avoided by preparing the chasm's main chamber as an arena beforehand, and choosing which Hearts to break first:
- For the first two, pick the least convenient hearts (distant, above the main chamber, underwater, etc).
- For the third, pick the closest to the main chamber, toss a Bomb or another explosive and quickly go to your arena while it falls.
If you have access to the Dryad, purchase several stacks of Purification Powder and completely convert the main chamber so that there is no Crimstone Block exposed to air, which prevents you from needing to deal with other Crimson-based enemies during the fight, although standard Underground enemies may spawn in their place. Set up several rows of platforms all the way across the central chamber, close enough that you can jump up to the next platform, and light up everything.
If summoned with a Bloody Spine, then the arena can simply be made in the Surface, however do not make it very tall, as jumping too far above it will be considered exiting the Crimson, causing the Brain of Cthulhu to despawn. If you have already prepared the central chamber as an arena for a prior fight, use the Bloody Spine there to avoid a despawn.
The Brain Of Cthulhu is unique in the way it reacts when you die. Unlike every other boss, The Brain Of Cthulhu does not despawn upon your death, rather it keeps ramming into the spot you died. You can use this to your advantage by using Purification Powder to clear out a small area to make a basic house. Then if you place a bed inside, you have a respawn point in the main chamber. Now, there is virtually no consequence to dying in the fight, as you can keep respawning and continue the battle.
Gold or Platinum armor is recommended if you are willing to mine enough of their respective ores. You can get Crimson armor by killing enough Creepers without necessarily defeating the Brain itself. Silver or Tungsten armor are slightly riskier, but ultimately also viable options, especially if you are confident in your dodging abilities.
It is possible to kill the Brain of Cthulhu with just Iron armor or Lead armor, but the fight will be a bit more dangerous. Pumpkin armor is another viable option, rewarding slightly less defense than Iron / Lead, but increasing your damage output, which can ensure the fight is completed quickly.
The Flinx Fur Coat is the only armor that can boost summon damage at this stage of the game.
Rocket Boots or its upgrades are recommended as they can help with dodging.
Accessories with Menacing or Warding modifiers will be most efficient here if you have them, but are not required.
For Melee users:
- Any large sword, such as the Blood Butcherer, will prove effective against the Creepers.
- A spear is a great weapon against the Brain of Cthulhu, especially during the second stage. Suggested options are the Trident and The Rotted Fork.
- A flail of some kind is beneficial for taking out the Creepers; the easiest to obtain would be the Flaming Mace (craftable from a Mace, which can be found in Gold Chests located underground). Even more effective would be The Meatball, which would require killing a few Creepers (but not necessarily the Brain itself). You could instead try to obtain the Chain Knife, rarely dropped by Cave Bats, but unlike the previously mentioned flail, it cannot pierce.
- A yoyo would be useful against the Brain in its second stage; the Artery is a great choice at this point, although the Code 1 (available from the Traveling Merchant) is also a good option if you can get one, having higher damage but quite low knockback.
For Ranged users:
- Tendon Bow is the most powerful bow at this stage. You can combine it with Jester's Arrows, which greatly help to kill the swarm of Creepers. However, you can also use Frostburn Arrows as a slightly cheaper alternative; the Frostburn debuff will be helpful during the second stage. Also, you can use Flaming Arrows as a fallback, with their On Fire! debuff also helping during the second stage.
- Blood Rain Bow does 5 less damage than the Tendon Bow but fires several streams at once. Its inaccuracy is offset by the many creepers to hit in the first stage and the large size of the Brain in the second stage. If you use this underground, make sure you're in a large enough space that the blood can fall freely from the top of the screen.
- Molotov Cocktails, while expensive to make (as Pinkies aren't common), is a superb weapon. The splash damage and the fire that hits multiple times can keep the Brain away in its second stage.
- If you are willing to risk, Grenades are great choices for dealing with the Creepers, but you have to watch out for the blast radius, as you will take damage from it.
- Non-explosive consumables, such as Shuriken and Throwing Knives, can be effective since they penetrate multiple enemies, meaning that they can hit multiple Creepers.
- Spiky Balls with their Penetration are very helpful for killing Creepers. Since they take a while to disappear, it's a good idea to throw many of them into your arena before starting the fight.
- As The Undertaker can be obtained very easily (as it's a guaranteed first-time loot from the Hearts), it's a good weapon to use against the Brain. The Musket Balls that spawn with it should be sufficient, but you can craft (or buy) Silver Bullets / Tungsten Bullets, which deal slightly more damage. Alternatively, once you obtain The Undertaker, you can gain access to the Arms Dealer, who can provide you with a Minishark, which is effective against the Creepers and the brain in the second phase if used with Silver or Tungsten bullets due to its rate of fire, and it will probably knockback-lock it, making tons of damage.
- Another gun that you might have acquired at this point is the Boomstick. It may be useful in taking out the creepers, as they have low individual health and there are many of them.
For Magic users:
- The Crimson Rod is a great support weapon that can easily be obtained through Crimson Hearts.
- A Ruby Staff (or, failing that, a Emerald Staff) is easy to make, and with its decent damage and piercing ability, it is particularly effective against the Creepers. A Diamond Staff or Amber Staff would work as well or better, but (being harder to obtain) are not worth going out of your way for.
- If you're willing to make a quick detour to a Corruption world, the Vilethorn works fantastically against the Creepers and is a great weapon in general.
- If you already farmed a Meteorite crash site for Meteorite, you can use the excellent Space Gun with Meteor armor: its piercing ability will clear out the Creepers fairly easily, and it will cost no mana thanks to the set bonus of Meteor armor. As of 1.4, meteorites now only spawn after the Brain of Cthulhu has been defeated. However, this is still a valid strategy if you have already defeated the Brain of Cthulhu at least once.
- The Demon Scythe is an excellent choice for dealing high damage to the second phase, if you're willing to get it.
- All these tips apply only for Desktop version and Mobile version only.
- The Snapthorn can help deal with the creepers and it will inflict the Poisoned on them for even more damage.
- For a minion the best options are Flinx Staff, Slime Staff and Vampire Frog Staff.
- The Ironskin Potion, the Swiftness Potion and the Regeneration Potion will do you well.
- If you're playing as a summoner, the Summoning Potion will increase your minion slot by 1.
- You should try to get the Well Fed Buff in some way; you can get this from Cooked Fish or (except on Desktop and Mobile) Pumpkin Pie.
- On the Desktop version and Mobile version, you could do even better with the Plenty Satisfied buff, which is easily obtainable from Pumpkin Pie, Cooked Shrimp, or similar.
- The Cloud in a Bottle will give you a double-jump ability which will greatly help with evasion. Look out for the alternative versions, some of which are more performant: the Fart in a Jar, the Tsunami in a Bottle, the Blizzard in a Bottle, or the Sandstorm in a Bottle.
- The Hermes Boots will increase your speed and aid greatly in evasion. You can upgrade them to the Spectre Boots for the added bonus of a few seconds of flight, but this requires the Goblin Tinkerer, which you may not have yet. Even better is the Lightning Boots, however they can take a while to obtain.
- The Band of Regeneration is a decent accessory which will slowly regenerate your life; it equates to one heart restored every 40 seconds. Not terribly useful but it's suggested if you have a slot you need to fill.
- The Anklet of the Wind gives a basic 10% speed boost, which will help out with evasion. You can replace it with the Aglet which is much easier to find but only offers a 5% speed boost.
- The Frog Leg will give you improved jumping abilities, including increased height and automatic jumping but it can only be found by fishing, though it is rather rare.
- If one is playing in Expert Mode, the Shield of Cthulhu is a great choice for quick evasion, and extra damage in the Brain's second phase. Specifically, if you dash whenever it disappears and reappears, you will either safely avoid the brain or run directly into it; either way, you won't take damage.