Each NPC in Terraria requires specific needs to be met in order for them to be at their happiest and sometimes for them to even move in at all. Happiness meters matter because it affects each NPC's price range from unhappy and pricey to happy and low pricing. One of the more unique NPC's is the truffle villager. He requires a fully complete man-made biome and is a talking mushroom. Much different than your regular humans and half humans that have taken up shop at your base.
It's tricky when it comes to getting the truffle to join the likes of your crew. He needs a Glowing Mushroom biome that is above ground and of the perfect block percentage and foliage. It can be annoying adding mud block after mud block to no avail, but it's easier than you think.
When You Can Get Him
First thing first is that you need to be in hardmode for sir truffle to even think about packing his bags. Every Terraria world, no matter which difficulty setting you choose when creating it, will eventually turn into "hardmode", meaning more bosses and more ore.
To reach this next section of your Terraria play-through you have to defeat the Wall of Flesh in the Underworld. He, or it, is summoned by dropping a voodoo doll of The Guide in the lava pits. You get these dolls from flying devils carrying them who drop it on death.
Most people usually prep before going to the Wall of Flesh first, though. There are other bosses finish off first, like every body part of Cthulhu himself. As soon as when the Wall of Flesh disperses into nothingness in defeat is how soon you can recruit the truffle NPC.
How To Get Him
Once your game is in hardmode comes the tricky part that most people have trouble with. You need to recreate the underground Glowing Mushroom biome, but above ground. You can do this on the main surface or you can build stairs into the sky and create it there, so long as there is mud beneath the house itself.
The house can be made of whatever your heart desires, but the ground must be at least 100 blocks of mud and lush with mushroom plants and grass. This is where it gets hard if you can't tell how many blocks you put down or you can't get your hands on enough mushroom seeds. There are two settings in the game that allow you to see block for block and count as you go. When you press ESC, there are three tiny icons to the top left of your inventory. The last one is titled "ruler off/on" and will show you a glowing ruler that helps you visualize block by block. Another setting is to press ESC, hit Settings, then go into Interface and make sure it says "tile grid" is on.
If you've traversed through jungle caves long enough then you probably have way more mud than you'd like, anyway. There's no harm in putting as much mud as your comfortable with in width and depth. To grow Glowing Mushroom foliage you need to plant mushroom seeds which you can get here and there when chopping down mushrooms in any mushroom biome that already exists. It's hard to get a lot of these seeds at once, but like any seed in the game, eventually, it will spread through the ground without you having to find exactly 100 seeds per 1oo block. As for the truffles home needs, it stays the same as every other NPC. Only the ground the house is built on needs to be exactly mushroom and mud. His house can be out of anything so long as it has a table, a chair, a back wall, and a light source.
What He Sells
Like other NPC's, every time you reach a milestone of killing a few high and mighty bosses, the truffle will have something new for you. Upon first acquiring him, he will have a Mushroom Cap, Strange Glowing Mushroom, and Dark Blue Solution for sale.
The Mushroom Cap is a vanity item, a hat, if you will. The next item grants you a baby truffle! Who doesn't want a mini truffle boy following you around everywhere? He also emits a little bit of light without actually being a light pet. Dark Blue Solution is used to create mana potions. After mechanical bosses have been killed, the truffle will start selling mushroom weapons such as a spear and a hammer. After you kill Plantera, he will sell a machine called an Autohammer that allows you to turn Chlorophyte bars into Shroomite bars. With this, you can create Shroomite armor and tools. The truffle boy will also sell Pylons when in the perfect home conditions and biome.