He is the patron god of the ruling class of Calormen. The Calormene capital is named Tashbaan and the Tisrocs and Tarkaans and Tarkheenas claim descent from Tash. The worship of Tash is the only formal religion depicted in the world of Narnia: there are temples to Tash, Calormenes regularly use ritual phrases such as 'Tash the inexorable, the irresistible' and 'Tash preserve us', and he is the only being referred to by characters as a god. At the end of the series, he appears as a terrible demon with a skeletal, humanoid body, a vulture-like head and four taloned arms.
The worship of Tash persists in The Last Battle, the final book of the series, in which he is depicted as a malevolent and real being. He is seen to be roughly humanoid, but much larger than a man, with four arms and the head of a vulture. His presence brings cold and the sickening stench of death. Illustrations by Pauline Baynes enhance his macabre appearance. It is said that the Calormenes practice human sacrifice to him. Narnians describe him as a god or a demon.
However, it is revealed that many of the Calormene invaders do not really believe in Tash. Together with Shift the scheming ape, Ginger the duplicitous cat and other treacherous Narnians, they concoct a story that Aslan and Tash are the same person, also known as Tashlan. Many (but not all) Narnians see that this is ridiculous, given Aslan's and Tash's antithetical natures, but are powerless to contradict the Calormene soldiers. Sending dissenters 'to meet Tashlan' in Puzzle's stable is meant to be a way to secretly murder troublemakers. But one Calormene soldier, Emeth, is so devout that he insists on going in to meet Tash, and vanishes into Aslan's Country (after killing the man with a sword in the stable meant to murder aforesaid troublemakers). Ginger finds Tash inside the stable, and is terrified into losing the power of speech. When Shift is thrown in, Tash appears and devours him. Finally, the Narnian King Tirian and Calormene warlord Rishda Tarkaan confront Tash. Tash seizes Rishda for having summoned him to Narnia, and is then banished by the command of High King Peter and the name of Aslan.
Emeth, who expects Tash to smite unbelievers with heavenly fire, goes searching for Tash in Aslan's Country, but instead meets Aslan. Aslan tells Emeth 'all the service thou hast done to Tash, I accept as service done to me' and further explains 'no service which is vile can be done to me, and none which is not vile can be done to him'. He explains that Emeth's pious devotion was really to Aslan, rather than to Tash, although Emeth had not been aware of this, and Emeth finds great happiness in this revelation.