Among Heracles’ Twelve Labors, the monster he exterminated at Lake Lerna was the Hydra, a giant swamp snake said to possess either nine or a hundred heads.
Possessing both a deadly poison said to instantly kill all humans it touches and an abnormally high regeneration capable of immediately regenerating two new heads every time one is cut off, the Hydra is a Monstrous Beast with top class fame even among the other Twelve Labors. Shishigou Kairi accepted a young-born Hydra as prepayment and processed it. At first, he processed it into a knife as a final weapon that could kill even a Servant, but later he used it to make an antidote serum out of wariness towards Semiramis.
Though Mordred blocked herself out from it with her armor, it still wasn’t enough to completely block the air itself which was contaminated with the poison. The rank of the Hydra’s poison is undoubtedly top A rank. It might even reach A+ rank against heroes of Greek myth.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia