The evil deity worshiped in Calormen is Tash. In C. S. Lewis’s Chronicles of Narnia, Tash is presented as a terrifying god associated with Calormene power, ritual, and sacrifice.
Calormenes invoke Tash in oaths and formal expressions, including “Tash preserve us” and “Tash the inexorable, the irresistible.” The capital city, Tashbaan, is named after him, and the ruling Tisrocs and Tarkaan nobles claim descent from Tash. His worship helps define Calormen’s authoritarian culture and contrasts with Narnia’s devotion to Aslan.
Tash is mentioned frequently in The Horse and His Boy, but he is not physically described there. In The Last Battle, he appears as a real demonic being with a vulture-like head, four arms, and taloned hands. Shift the ape tries to merge the names of Aslan and Tash into “Tashlan,” but the story ultimately presents them as opposing figures. Emeth, a sincere Calormene worshiper, discovers that his devotion has been directed toward the wrong object when he encounters Aslan.