Mercurial Essence is included in The Alchemist Potions.
This Essence is one of the most delightful ingredients to behold in the apothecary. Shimmering an shingles’s, it looks like it is transforming before the beholder’s eye - and in fact it is. The liquid is constantly changing properties and can be a bit of a fussy ingredient to work with.
Derived from melted down metals from middle earth, Mercurial Essence strength can change based on the maker, source of metal and time of year it was made. It might take some experimenting with the amounts required to make potions effective, and if you potion is not working it could be related to your Mercurial Essence. Investigate and experiment!
Luckily, as it sometimes take lots of the ingredient to get it right, you can refresh your supply by adding water to a bottle close to running out.