The giant poisonous snake summoned by Semiramis, Assassin of “Red,” with the power of “Sikera Ušum: Arrogant King’s Alcohol.” It is one of the eleven demons birthed by the monster of Babylonian legend, Tiamat. It is a species of Divine Beast, the top of all Phantasmal Species, but even Semiramis wouldn’t be able to summon something so powerful in that rushed situation, so she only summoned its top-half.
If Shishigou hadn’t used a Command Spell on Mordred, she would have immediately died the instant Bašmu opened its mouth and released its poisonous breath.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia