The toraline-based ink that borrows spells from the willing and steals them permanently from the unwilling.
Edgar describes the ink Carl had, made with Toraline: it allows one to borrow spells and skills temporarily from willing subjects, and to take them permanently from unwilling subjects.
With help from Milk, Edgar has developed Essence Ink, similar to this ink but not of the same form, capturing a fallen crawler's essence instead. Because he extracted Bijanbi properly using Milk's ink tips, Rosetta notes they no longer need toraline to do what the plan requires.
The predecessor ink whose limitations the new essence ink overcomes.