ON THIS WIKI
Corrosive Venom is an enchantment to Ethereal Familiar, which will inflict Poison to enemies it strikes. It will double its efficiency for each level upgraded, on 1st level it will poison for 6 seconds, on second - for 12 seconds and on third - for 25 seconds. In order to upgrade the Familiar with a certain level, it has to be already upgraded with the previous level of enchantment.
Thaumcraft 4 Research Notes
Somebody call the doctor
"A familiar with this upgrade will inject a large dose of highly concentrated venom into its victims rather than doing damage, inflicting them with severe poisoning. Familiars with this upgrade seem to be able to detect which foes can be poisoned, and as such, it will only attack those that can be poisoned.
Consumes additionally 0.03 Aqua and 0.04 Perditio Vis per attack and level."Aspect the Ethereal Essence is related to has to match the Aspect the Ethereal Familiar is related to and the 4th aspect in the recipe. In this article it is
Corrosive Venom I
Corrosive Venom II
Corrosive Venom III