Terraria: Spawn Mechanics

Farming for Mimics, Weapons, and Materials

Something new players won't realize immediately is that the spawns in Terraria are based on blocks. That simple, though spawn rates are a little more complex. You can completely control this process and ensure that enemies spawn in the correct place, on your terms, and use traps or hazards like lava against them. You can additionally speed up manual farming by simply making an area where enemies spawn in predictable places so that you can take them out fast, allowing more to spawn in their place. These are the best ways to get rare accessories and weapon drops - for example from mimics - in Terraria.

If you are lazy and don't care about all this mechanical stuff, you might skip to Putting it Together but at least check my images for tips.

Spawn Mechanics
The wiki has some insight into this, but I will tell you how to apply it and simplify it. When the game decides to spawn an enemy it looks for a nearby block that is on the ground, chosen randomly. Even flying monsters seem to spawn from a block, unless they're harpies and certain others. The type of enemy is based on the type of block available for spawning, based on a zone that is off screen. An enemy from the biome that block comes from will spawn just off screen, not even 1 and a half screens away. Their AI then tends to send them toward you. With that, we can make a meat grinder.

In many cases the block decides the enemy that spawns, but Biome is also important. So once you have blocks set and determined the biome of the area, enemies that are exclusively spawned by biome can spawn. If you want to Farm Mimics then you want a Hallowed/Crimson/Corruption Biome built. If you want to avoid them, then do not include these Biomes.

Spawn Zone
Enemies spawn in a rectangle around you, no matter your resolution. They're always off screen unless you're using a resolution-increasing mod to go beyond 1920x1080, it's why we don't see monsters appear out of thin air. The screen area (no spawn zone) is 62 tiles to the left and right of the player. 35 tiles up and down from the player. Monsters can spawn in an area 22 tiles to the left and right of this, and 12 tiles up and down - roughly. This means there's room for error, but it's also not a very large zone where spawns actually take place. If monsters are too far outside the spawn zone, they despawn.

It is hard to imagine the size, but players on PC have access to the Mechanic NPC's Mechanical Ruler. Place a torch and count every 10 blocks from it. When you hit 70, enemies will spawn outside this zone. Do it for both sides. You can dig out an area of sufficient size, deep underground in the Cavern layer to ensure the highest spawn rates for the biome you need to farm. Find a middle ground and dig it out, then count the appropriate number of blocks' distance.

Spawn Rates are based on Depth and Biome
The rate monsters spawn are based on a few things - but for our farming purposes mainly depth and the type of Biome. Biomes like Jungle and the Underground Crimson/Corruption have higher spawn rates than a normal "virgin" cavern-layer area, and far more than the surface aside from it being in Event (Pumpkin Moon), Blood Moon, or Solar Eclipse mode, which have set rates and cannot be manipulated. Generic biomes like underground ice, desert, marble and granite biomes follow the Underground Cavern spawn rate and are not as dangerous as that of the Corruption. You'll likely notice this while playing.

The types of enemies that spawn and the drops increase at the Cavern depth, so if we're in late Hard Mode we'd want to set something up deeper underground to get the really good stuff. Mimics from the Hallow and Corruption can be spawned manually (15 soul of light/night turned into a key and placed in a chest), but we can make them spawn automatically more frequently and get their drops.

Putting it Together for a Simple Manual Farm

As the guide stated above, enemies will spawn anywhere from 64 to 82 blocks away from your character horizontally, and 35 to 47 blocks above and below the character. To truly have complete control over spawns, we'd want an area about 170 blocks wide and 100 tall, void of blocks. We will fill in a line across at the center. Because of digging, this is easier to do on the surface, but it's not always best if you're hunting Hallowed/Crimson Mimics because of the spawn rate being higher underground. This size will ensure control over spawns for the most part. You can help yourself measure by starting on the right/left edge of your desired zone and working your way the opposite direction with an empty stack of blocks. If you peel 180 blocks off the top, you've got yourself a good farm zone.

Now go to the edge and count 100 down from the top. From there, hollow this area out and rock on! It doesn't need to be exact and you can be safe to go a bit bigger than this.

Dig down 3-5 blocks, then move across, then repeat this until ALL blocks are gone. One stray block can cause monsters to spawn below/above you and be the wrong type. It shouldn't take more than 10-15 minutes but the rewards are worth it. There's a good reason for digging it out - it's easier than filling in blocks, especially with a Chlorophyte+ Pickaxe - and even better a Shroomite Digging Claw! If you were to try to fill them in, you'd have to make sure there is 0 space for monsters to spawn.

Determining Biome & Making a Monster Farm

As I said in my page about making artificial Hallow & Corruption/Crimson biomes, the only thing that determines a biome is the quantity of blocks around the character. Some take priority over others, while some are compatible (think red ice blocks from Crimson spreading there, yet still spawning ice monsters along with Crimson monsters). We can make totally pure biomes that spawn only what we want, and where we want by digging out an area.

Lay a layer of stone - or Crimstone - or Pearlstone across the area you want to farm in. Form the foundation. Once it's all dug up, you can change this at will by simply changing the block type on top of the stone. If you frequently need souls of light or souls of night, then make it Hallow/Corruption stone for a base so that it counts as a hybrid biome. You'll get Souls of Night from, for example, Ice Tortoises while still getting a chance at a Frozen Turtle Shell.

However, you'd get Crimson/Hallowed Mimics because they are spawned by the biome your character is in. You may instead want Ice Mimics - in this case, you'd want to go 'pure' or ensure there are so many snow/ice blocks in the central area that it counts as an Ice Biome while making the blocks enemies spawn from be Crimson (a thin layer of crimstone on top is all you need for this)..

Determine the Biome & Spawns
Build upward from the center, making a column so that you can place blocks midair. Now make a 200-400 floating rectangle segment above the central area so that your character is counted as being in a frost, desert, etc. Biome (as pictured above). This is all it takes - the background should change. The monsters that spawn will be based on the blocks you've laid in the 'spawn zone' off-screen. So, if you are hunting Hallowed Mimics for the Daedalus Stormbow, then lay Pearlstone blocks above your character so that it counts as a Hallowed Biome.

Boost Spawn Rates
I already have a guide to Water Candles and Battle Potions which will nearly double the amount of monsters you encounter. The water candle is easily doable - just grab one from the Dungeon with a pickaxe or craft one if you're on PC (the first is easier). If you'd like to do things more automatically, stay tuned as I have simple guides coming that will cover making wastelands that spawn only monsters you desire based on biome, and even keeping them where you want them so they are easy to kill via trap or your own blade.

Best Spawn Rates & Farming Biome Keys
The best spawn rates are found in Jungle and Dungeon Underground Biomes (Cavern layer). However there are no Jungle Mimics (yet) and you will not get Crimson/Corruption/Hallowed Mimics with this setup alone - however you can farm regular items very well. You need Jungle Grass/Lihzard Blocks or Blue Blocks from the Dungeon in order to make that the biome you're in, while the spawn blocks you use are of the type you want to farm. The Frozen Turtle shell is a good item, but you may also need biome keys from a specific biome.

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Getting Chances of All Biome Keys and Mimics at Once
You can farm Mimics and Biome Keys/Key Molds using these methods by combining Biomes. Farming Biome keys all at once can be accomplished by having enough blocks to create each Biome whether the background changes to that biome or not. So if we make our main Biome Jungle (with the 80 required Lihzard/Jungle Grass blocks above/under the player) but also have 200 Crimson, Hallow, and Ice Blocks, we can have a chance at all of those dropping at once. If you want the biome to be mainly jungle, get as many Jungle Grass around your character as possible so as to override the others. You can tell when the music changes that you're benefitting from the increased spawn rates.

By placing a Water Candle on a table and consuming a battle potion every so often, enemies will spawn fast - you can take them out with a ranged weapon (fastest) as they come toward you. Slaughter them and take the loot every now and then. It's surprising how much gold and materials you can farm doing this, and how regularly you'll get Mimics. Once you know about these mechanics, you can set up your own automatic farms that are super efficient and allow you to leave the computer while they run. See below.

Having Fun & Scoring Loot With AFK Farms

Joe Price, Zerogravitas, and HappyDays are names you should know if you're interested in automated farming. Click to learn more as I've linked to all three of their channels on Youtube so that you can see how things are done. For many of the builds you will learn of, these mechanics come into play whether above or underground. However, I still suggest you play around with them yourself and use them to farm the accessories you need.

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