A Magic Formula is a quest in The Witcher, received in Chapter V if Geralt chooses the Witcher / neutral path, sends the witcher to the southernmost reaches of the swamp cemetery, to a refugee cave where he must retrieve a magical formula from an ancient crypt to help Kalkstein re-create Raven's armor.
After finding out that Kalkstein can indeed craft armor, given the right components, Geralt heads on out to the southernmost refugee cave in the swamp cemetery where the alchemist has told him he will find a Wayfarer's stone permitting entry to an ancient crypt. This cave also happens to be the very cave where Zdenek, the most famous fist fighter in history lives (pretty humble accommodations!).
To gain entry to Agnes of Glanville's lair, Geralt must use a specific combination of signs which he has conveniently given by Kalstein: Aard, then Igni, then Aard again to activate the stone, and poof! He is in.
Once inside, he is greeted by alghouls, fleders, cemetaurs and basilisks in this, the largest of the three Ancient crypts. After battling his way through to Agnes' primary chambers, he will find a chest with the magic formula that Kalkstein has requested. Then it is back to the alchemist with the formula.
Elements of the Armor
Kalkstein told me to bring him the pieces of both Raven's armor and the armor of a knight of the De Ruyter dynasty, the notes of an elven minstrel, and Agnes of Glanville's magic formula. I'll find all these items in the swamp cemetery. I can access the sorceress' underground laboratory through a cave where some refugees have found shelter. Once inside, I need to find a stone sensitive to magic. A combination of Aard, Igni, and Aard Signs will open the passage. I must go to the swamp cemetery and find all the elements required to make the armor.
I have the magic formula. I need to deliver it and the other items needed for the armor to Kalkstein. I need to deliver the magic formula and the other items to Kalkstein. (9000 XP)