Mól alchemy
The Empire's alchemical learning, held now only in the ciphered volumes of the Mól Encyclopædia
Alchemy as the Empire practiced it, and all that is left of it is in cipher. It comes out of the alchemy volumes of the Mól Encyclopædia and out of no other source. Mikhail broke the first of those volumes in a cursed keep in S2E19, and what the volume gave was not a recipe but the subject itself. A reader who has it can attempt work on Mól alchemy that a reader without it cannot begin at all.
The gate is the language before it is the learning. Mikhail took the volume to Gwenivere for help with the cipher, and she could not touch it: she reads a great deal, but she does not read Taal. He translated it aloud to her instead (S2E25). She sent the party on to Ashanti, whose own trade this is, and the alchemy volumes have gone to her since.
Clan Saytar were the Empire’s alchemists and are named as the craft’s seat. Their laboratories have never been found, and what the Empire did with the work at scale was never written down.