Seihitsu no Hassan, known as Hassan of Serenity, is an Assassin-class Servant in Fate/Grand Order. She is one of the many individuals to bear the title Hassan-i-Sabbah, leaders of the legendary Hidden Hassan order. Seihitsu no Hassan is a quiet, gentle girl whose body is laced with deadly poison, making her touch and presence lethal to others. Despite her lethal nature, she yearns for human warmth and connection, and her tragic role is highlighted through her loyalty and deep affection towards her Master.