To progress in “Delving into the Eldritch”, you’ll need to obtain something native to the void, void seeds. You need the Causality Collapser which has the “Flux Rifts” and “Flux” research as prerequisites, found on the “Fundamentals” tab.
The Causality Collapser is a device that when thrown (as you would a snowball) into a rift, closes the rift. The Thaumonomicon recommends that you first stabilize the rift.
After the rift collapses in either case, you will get void seeds (about 10) and a chance at a primordial pearl/mote.
Before we can unlock the Eldritch tab you will need to do a little research, this can be made easier if you place a Brain in a Jar near the Research table. You will also need to fight a few Crimson Clerics (the guys in the robes) as well as Scan both types, this will unlock The Crimson Cultist Research and The Crimson Rights Research, they will also drop Crimson Rites, this book cannot be consumed or read yet but it will be used for research:
The brain in the jar will also give you Eldritch research which you should focus on:
Once this research is done it is time to find some Cultists.