Manuscript page: Hanged Man's Venom is an alchemy formula in The Witcher 3: Wild Hunt that is needed to craft Hanged Man's Venom.
It is guaranteed to appear in tinderboxes in the basements at Keira Metz's hut and in Mowshurst, right next to the corpse where Out of the Frying Pan, into the Fire starts. It can also be found at random diagram locations or purchased from the following merchants:
In the unlikely event that it hasn't been obtained until that point, it can also be given as a reward from Eskel for beating him in a horse race at the end of To Bait a Forktail...