This is considered the “end-game” of Thaumcraft. You are given the ability to craft high-tier weapons, armor, and tools, at the cost of your sanity. Researching any topic in this tab will require Eldritch research alongside gaining warp when you unlock it.
To start with the eldritch tree, a crimson cultist needs to be scanned. I found this easiest by running around with high warp (ate half a stack of zombie brains from my zombie farm) in a desert (good visibility). A red and purple portal will spawn and out will pop some cultists. The mages have seeking projectiles so watch out for those. After scanning a cultist, you will need one of the books they have a small chance to drop. You may need to encounter a few portals before one drops.