Fate/Grand Order Absolute Demonic Front: Babylonia Season 1 Episode 20
Absolute Demonic Front Mesopotamia II
Tiamat falls into the underworld, but there, she transforms into one of the Evils of Humanity: Beast II. Hassan-i Sabbah, the first Old Man of the Mountain, intervenes, using his power as Grand Assassin to deprive Beast II of her immortality.