The ingredient in Carl's old tattoo ink that permits borrowing or stealing spells and skills from other beings.
Edgar recalls that the ink Carl had, made with toraline, allows one to borrow spells and skills temporarily from willing subjects — and to take them permanently from unwilling subjects. It was the basis of the earlier Blood Tattoo Ink work.
With Milk's help, Edgar developed an ink similar to the toraline version but not of that form, one that captures a fallen crawler's essence instead. Because Edgar extracted Bijanbi properly using ink tips from Milk, Rosetta notes they no longer need toraline to do what the plan requires.
The obsolete ingredient the new essence ink replaces.
Also appears in books you've already read — spoiler-safe to their endings: