Plantera

There is no summoning item for her. She does not exist until all three mechanical bosses are dead, and the Wall of Flesh has to have fallen as well, since the message announcing that the jungle has stirred only fires once Hardmode has properly begun. From then on, pink bulbs start growing on the walls and ceilings of the Underground Jungle, glowing faintly in unlit areas. Break one and she arrives immediately, normally from somewhere just off screen, so hitting a bulb is the same thing as starting the fight.

Only one exists at a time, and smashing a second bulb while she is chasing you wastes it. Bulbs keep growing after she dies, so you can refight her freely, and they will also grow in a jungle you planted yourself, which is the cleanest way to pick your own battlefield.

She creeps instead of charging, dragging herself along on three grappling vines that grab solid blocks. The claws on the ends of those vines cannot be destroyed and hurt on contact, so there is no safe side to stand on. She deliberately keeps her distance and shells you from range, and she only fires when she has a clear view of you, which means terrain genuinely helps in this half.

It starts as plain seeds. Once you have taken a decent bite out of her health, poisoned seeds and thorn balls join in. The thorn balls are the real nuisance: they bounce off surfaces quickly and erratically and drift toward wherever you are standing horizontally, so parking in one spot slowly fills the floor with them.

At half health the petals drop away, a mouth full of thorns opens underneath, and she abandons her projectiles entirely. Speed replaces them. She comes straight at you for contact damage, which now hurts considerably more, though her armor collapses at the same moment so your weapons finally cut through.

Eight mouths sprout on vines from her body. They deal contact damage but their real job is soaking the shots you meant for her. They die fast, and in a normal world they stay dead, so clearing them early makes the rest of the fight much simpler. She also starts releasing drifting pink spores, which die to one hit from anything but pass through terrain and hurt badly. Since they are her only ranged attack in this half, distance keeps you alive rather than cover.

Take her to the surface, down to the Underworld, or into any unrelated biome and she enrages. Her damage doubles, she becomes much harder to hurt, she moves faster, and she throws away the careful spacing to rush you down. Her projectiles hit harder and come out sooner too. Bringing her back underground calms her again. Corrupt, crimson or hallowed patches inside the jungle do not count against you as long as enough jungle grass, hive or temple brick is nearby, so a spreading world does not take your arena away.

In the first form her seeds gain the ability to pass through blocks and every projectile picks up limited homing, which kills the hide behind a wall plan outright and forces real dodging. Cooldowns shorten as well.

The second form gets worse differently. Nine more mouths appear on the grappling claws themselves, everything moves faster, and the eight on her body grow back after you kill them. Regrowth is slow while most are alive and quick once they are all gone, so grinding them down repeatedly is wasted effort. Clear whatever blocks your line and go back to the flower.

A large open box beats anything clever, and explosives like Scarab Bombs dig one far faster than a pickaxe will. Hollowing out an existing hive is a common shortcut, though the larva inside will summon its own boss if you are careless. Strip the background walls out: without them she has nothing to hook onto in her second form and her mobility falls apart.

• Leave a deep empty pit under the fighting area so thorn balls rattle around well below you, or drink a Gravitation Potion and let the ones on the ground fall away instead.
• Light the place with Living Fire Blocks rather than torches, since she cannot grab those.
• Narrow passages slow her because she has to keep finding new grip, useful if you need to break away.
• A hellevator plus wings works if you would rather not dig: pull her near the top of the jungle layer and glide down while shooting.
• Trapping her is pointless. Her hooks pass through blocks, and with nothing to grab she simply floats upward, eventually all the way into space.

She always leaves the Temple Key, the only way into the Jungle Temple and the road to the Golem. The Dungeon then turns over to a much nastier enemy list, and the spirits those enemies leave behind drop Ectoplasm, which is what unlocks the two moon events. Biome chests become openable. The spread of corruption, crimson and hallow drops to half its old rate, which is real breathing room for a world that was getting eaten.

• The Cyborg can move in, which in turn makes the Princess possible.
• The Truffle starts stocking the Autohammer, and the Witch Doctor adds Leaf Wings and a pile of summoner gear.
• The Zoologist sells the Mud Bud, and the Dye Trader starts paying out new dyes for strange plants.
• Solar eclipses gain a much harder roster, and the Prismatic Lacewing begins appearing in the surface hallow, which is how the Empress of Light gets called.

One quiet detail: with no Lihzahrd Brick or temple wall left anywhere in the world, she drops temple building material on death. Demolishing the place is how you recover a lost Lihzahrd Altar without a new world.