Mortificatio Oil
No known way to obtain it. The game's data has no way to get it.
The Mortificatio Oil is in the game files, but nothing in them gives it to the player: no recipe, merchant, quest reward, container, battle reward or script.
It belongs to a set of twelve alchemical stages with the same fate, from Nigredo Ash to Rubedo Crystallis. None of them has runes in the mixing table, so the Laboratory would not take them even if you had one.
No known source in the extracted data.