Guide:Farming Biome Keys or Key Molds

The Biome Key Molds can provide the player with some of the game's most powerful weapons. As such it can be very convenient to obtain them in a relatively quick manner. This article attempts to list methods of doing so.

Events
Events (aka invasions) are an excellent way to farm key molds. They provide large numbers of monsters as well as coins and interesting drops on their own. As some events occur by random and others by using items, players who don't feel like waiting should gather items for the latter type. Events may also be chosen out of their drops. Below is an overview of events:



Defeating events may additionally require various setups to make the fights easier, largely depending on the event you choose. Pirates and Goblins require next to nothing, while the Pumpkin and Frost Moon usually need elaborate arenas.

AFK farming
The time it may take for Key Molds to drop makes AFK farming an option some players may consider. There are three things that can kill monsters without player interaction: minions, 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 the character should however be walled up as protection. The "waiting room" should be some height above the ground to avoid enemies that can pass through blocks (Wraiths, Devourers), and protected by traps to prevent flying enemies from piling up. 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 (list). 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.

Arena tips
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.
 * 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.
 * If you start with a Mud Block-based biome, be sure to farm the Jungle Key Mold first! When you transform it into the Corruption or Crimson (or Hallow, after either of those) the top block layer will be transformed into Dirt, on which Jungle cannot be re-applied.
 * 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:
 * Guide:Base defense and precautions
 * Guide:Containing the Hallow/Corruption
 * Guide:Defending against events

Corruption 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 1/50 (2%) drop rate for a key mold from the Eater of Worlds. With Hardmode equipment, spam summoning the EoW is quick and reliable.

Another, lesser, method is to kill worm type creature like the World Feeder with explosives. This will kill multiple segments at once, each dropping their own loot.

Crimson 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. Spam summoning the Brain of Cthulhu is not as reliable and fast as the corruption equivalent, but still convenient.

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 as made for this purpose. In 1.2.3 they were however changed to not drop any loot once in Hardmode, removing them as a means of farming.