Thaumcraft adds a new biome to the world - Magical Forest. You can’t miss this luminous green biome. Within the Magical Forest there are higher concentrations of vis (great place to set up a base). You will also find vis mushrooms, greatwood trees and silverwood trees.
Within the forest you will also find pechs (pronounced “pecks”). These creatures are neutral and will not bother you, but they will pick up items you have dropped. You can also trade with them if you give them enough items containing Desiderium.
After you have thrown the pechs enough items containing Desiderium, you can right click them and add items containing Desiderium in the left most slot. By clicking the die in the middle you might get something in return.