Pact ink is the specially prepared ink that a contract with the devil must be written in, otherwise the contract is invalid. Various recipes exist. Detailed instructions are rare. The basic recipe is the same as that for Iron gall ink.
Pact Ink for Devils and Spirits (14th century)
Arthur Edward Waite's Book of Black Magic gives a similar recipe, but with directions.
References
Pact ink Wikipedia(Text) CC BY-SA