This means the page will walk you through a specific task, strategy, or enemy/boss fight.
A dropped Hallowed Key Mold.
Biome Keys are used to open Biome Chests in the Dungeon, which provide access to some powerful post-Plantera weapons, such as the Vampire Knives and Rainbow Gun. However, Biome Keys drop at the very low rate of 0.04% (1/2500), and are only dropped when the player is in the key's corresponding biome. Thus, it is advisable to set up an efficient farm to obtain them.
On the 
console, 
mobile, and Nintendo 
versions, Biome Key Molds drop in place of Biome Keys, which require drops from Plantera and all three Mechanical Bosses in order to be crafted into a Biome Key. This can cause additional strife if the player is lacking any of those materials, so it is helpful to keep a healthy stock of them.
Events
Events are one of the best ways to farm Biome Keys and their Molds. An extremely large amount of Enemies spawn during events, and many of them will have a chance of dropping a Biome Key or Biome Key Mold, presuming you are in the biome in which one will drop. In addition, farming events provides a great opportunity to get rare event drops.
This is an example of an Arena which can be used to fight Events and farm the Hallowed Key or Hallowed Key Mold, because of the Pearlstone Blocks below it.
Some events the player may want to consider using to farm:
- Goblin Army — The enemies that spawn at this stage of the game shouldn't be an issue for the player to deal with. However, if you are playing on versions later than 1.3, you may want to take into account the Shadowflame Apparations the Goblin Summoner may summon if left unattended.
- Frost Legion — There are few enemies in this event capable of attacking through walls, but be wary that Snow Ballas may nullify an arena's effects (most notably, lava) if they aren't dealt with immediately.
- Pirate Invasion — The summoner is easy to obtain, and most pirates are limited to attacks that can't penetrate blocks. However, at this stage, it is likely the Pirates aren't incredibly easy to take down, so you should maintain constant damage output.
AFK Farming
The time it may take for Key Molds to drop makes AFK farming an option some players may consider. There are a few items that can kill monsters without player interaction— minions, accessories, armor (turtle, stardust, solar), lava, and traps. The two latter are often used to make arenas for defeating events, meaning an arena can be convenient for AFK farming. In fact, if the arena is good enough it's even possible to use events for AFK farming. Without player supervision, however, the character should be walled up as protection. The "waiting room" should be some height above the ground (so the ground is off-screen) to avoid enemies that can pass through blocks (Wraiths, Devourers), and protected by traps to prevent flying enemies from piling up, and increasing spawn rate. The event should also not feature flying enemies that can pass/attack through walls, such as Elf Copters.
If the player doesn't have or care to make an arena, it's alternately possible to just use minions. This approach largely requires the player to use minion-empowering items. As minions are able to pass through walls when returning to the player, but unable to when attacking an enemy, no "waiting room" should be constructed.
Keep in mind that even while AFK-farming, the player is still required to check the progress once in a while. Otherwise any dropped key molds will disappear due to the aforementioned note.
A player using Ravens to farm.
A character sitting inside a "waiting room" while traps are killing monsters.
A character using an oceanic key farm with a lava trap to kill monsters.
Arena tips
A piece of land transformed into the Hallow for mold farming. The surrounding shaft prevents the Hallow from spreading to the adjacent land. Notice the platforms which allows monsters that spawn in or fall into the shaft to climb up.
If you want to artificially build a biome to farm Key Molds in, there are two ways to do so:
- Gradually stacking one or more block types of the relevant biome until the amount of blocks calls it forth. The compactness of the stacked blocks allows a relatively small area to create a biome, which can be convenient if you're making an arena which doesn't allow for things like grass and natural cliffs - instead, one can just stack the blocks below the arena. For info on what it takes for a biome to form, see Guide:Artificial biomes.
- By using 200 blocks of Red Ice, 200 blocks of Purple Ice and 80 blocks of Lihzahrd Brick it will be possible to have 4 biomes overlap and all 4 biome keys to drop. Hallow is unfortunately exclusive with Crimson and Corruption, and will require a separate arena from those two.
- Simply transforming a default biome into another. The Clentaminator is an excellent tool for changing biomes, also doing so stunningly fast. It's also possible to use Grass Seeds if the player doesn't mind waiting.
- Unless you don't mind the Corruption, Crimson, or Hallow spreading, you may want to "quarantine" the area by digging a three-block-wide shaft and tunnel to separate the Key Mold-biome from the adjacent land.
- A Water Candle will increase the number of enemies. This can also be combined with the Battle Potion.
- A Campfire, Heart Lantern, and standing in Honey will increase a character's survivability.
- Additional info on creating biomes/arenas can be found on:
- The dungeon has a high spawn rate to start off with, so setting up a farm there can help speed up the farming process.
- Actuators can help with getting in and out of your waiting room while blocking enemies from attacking you.
Mold-specific farming
Corruption Key/Key Mold
- The Eater of Worlds's body segments all count towards the drop rate of 1/2500. Since the Eater of Worlds consists of 50 segments there is a higher drop rate for a key mold from the Eater of Worlds. With Hardmode equipment, spam summoning the EoW is quick and reliable.
- While farming, remember to return to the Dryad if a Blood Moon occurs, as she will sell Vile Powder, which is one of two ingredients in making Worm Food. The other being Rotten Chunks. This prevents the player from having to farm Vile Mushrooms, but they function as an alternative if waiting isn't an option.
- Another, lesser, method is to kill worm type creatures like the World Feeder with explosives. This will kill multiple segments at once, each dropping their own loot.
Crimson Key/Key Mold
- The Brain of Cthulhu uses a group of creatures called Creepers to attack the player. Each Creeper has a 1/2500 chance to drop the Crimson Key Mold, and spawn in groups of 20 to 30, meaning the average drop rate per Bloody Spine is about 1%. Spam summoning the Brain of Cthulhu is not as reliable and fast as the corruption equivalent, but still convenient. Crimson Key Molds are used to open the Crimson chest, which contains the Vampire Knives, arguably one of the best lategame melee items.
Hallowed Key/Key Mold
- This may not be the fastest strategy, but standing on a hill in The Hallow and using the Spectre Staff to kill any enemies that come into your field of view is somewhat effective.
- A more AFK equivalent is to stand on the hill and use a Pygmy Staff along with some sentries to automatically kill the enemies. Turtle Armor can help do damage even if they get to the player, and Campfires and Heart Lanterns help boost regeneration.
Trivia
- Prior to patch 1.2.3 it was popular to use Meteor Heads to farm key molds. Being summonable with enough Meteorite, easy to defeat, and posing hardly any danger to the player, Meteor Heads were seemingly made for this purpose. In 1.2.3, however, they were changed to not drop any loot once in Hardmode, removing them as a means of farming.
- BoxFarm.3.PNG
Meteor Head farming with traps.
- PygFarm.PNG
Meteor Head farming with Pygmies
- Aviary.png
Meteor Head farming with Ravens.
- Easyfarmingkeyssetup.PNG
Meteor Head farming using Turtle armor's Thorns Potion-like effect.
