Brain of Cthulhu

There are two ways to bring it out. Breaking the red hearts buried in the chasms counts up, and every third one you shatter drops the boss on top of you immediately, which is how most first encounters happen: mid tunnel, pickaxe in hand, no arena. The deliberate route is a Bloody Spine, used while standing anywhere on Crimson ground, surface or deep underground. Time of day does not matter and nothing else has to be beaten first.

Leaving the biome ends it. The boss sinks downward and vanishes, though if you sprint back inside quickly enough the fight picks up where it left off. Smashing hearts carelessly can also land you with two of them at once, sharing a single pool of escorts and flipping into their vulnerable state together. You can call it back as often as you like afterward, so a botched attempt costs you one spine and nothing else.

The boss cannot be hurt at all to begin with. It fades in and out around you while twenty escorts do the actual damage, and those escorts never come back once killed. That gives the phase a fixed length: it ends when the twentieth one dies, however clumsily you get there.

The way they move works in your favor. They sweep around the boss in loose orbits, then fling themselves at you and get dragged back, which packs them into clumps on every return trip. Anything that pierces a line of enemies or splashes on impact clears half the phase in a few well timed hits, and Jester's Arrows or a grenade do more here than a weapon with a better damage number. Burn and frost effects finish whatever survives, since they die to almost nothing. They also cough up hearts far more often than ordinary enemies, so a sloppy opening frequently leaves you at full health for the real part.

When the last escort falls, the shell splits open and the core inside becomes a target. It stops being passive at that point: it charges you, blinks around at speed, and takes damage and knockback like an ordinary enemy. It also disappears from the minimap, so tracking it that way is not an option.

In Expert and above, three mirror copies appear around you for the second half and repeat everything the real one does, growing more solid as its health falls. Several things cut straight through them. Only the genuine one carries a health bar. Only the genuine one takes burn, frostburn or poison ticks, and only it shows the visual effect. Minions and sentries never chase a fake, Bone Javelins stick only in the real body, a slime gun paints only the real one, and controller lock on ignores the copies entirely.

There is a movement tell as well. The copies position themselves relative to you rather than to the boss, so walking steadily away fans them out while the real one keeps its own approach. Escorts in the first half also charge harder and faster at this difficulty, and contact with one always leaves a random debuff behind, while the boss itself only manages that occasionally.

This is the one boss in the game that flinches. That changes what a good weapon looks like here, because something with real push behind it holds both the escorts and the core at arm's length between teleports in a way that raw damage cannot. The Rotted Fork or a Trident will do more for your survival than a heavier hitting sword that shoves nothing, and a Minishark covers the second half for the same reason. The catch is that once illusions are involved you often cannot tell which target you are pushing.

You do not strictly need the kill to move on. The escorts alone drop enough ore and samples to build the gear tier, and skipping the boss that way is a legitimate route. Finishing it is what unlocks the rest.

• The Tavernkeep begins appearing in the world as the Unconscious Man, waiting to be found and freed.
• Meteorites can start landing at night, and each further kill improves the odds of one arriving.
• The Dryad will move in if she was holding out, and stops talking about the chasms.
• The Merchant, the Arms Dealer and the Traveling Merchant each open up new stock, and the Angler adds two more fish to the pool of things he can ask for.
• In Expert the treasure bag holds the Brain of Confusion, a dodge accessory that keeps earning its slot long after the rest of your pre-Hardmode kit has been retired.
• If your world generated with no dungeon bricks or matching walls anywhere in it, the kill hands over a set of building materials to fix that.

These seeds rewrite the fight rather than scaling it. Forty escorts instead of twenty, charging far more often and accelerating much harder into each dive. The boss skips around three times faster during the opening. Its defense climbs high enough that most weapons you could reasonably own at this point barely scratch it, which turns the second half into a long grind. The illusions go fully solid the instant you land your first hit, at every difficulty, so the tells above become the only way to find the real one. Worst of all, gravity flips the moment the second half begins and no potion or accessory will flip it back.