Rishda Tarkaan was a Tarkaan of Calormen who lived during the last days of Narnia.
At first he helped the ape, but, along with Ginger the cat, grew tired of the ape doing everything. Between himself and Ginger they devised a plot to make it seem as if "Tashlan" was inside the stable, because the animals were getting tired of not seeing Tashlan. Ginger was to go into the stable, and then come out saying that he had seen Tashlan. Rishda had also positioned a Calormene sentry to kill any talking animals that went into the stable, but he had orders to not harm Ginger.
When Ginger went in, and came out terrified and unable to talk, having actually seen Tash, Rishda's plan began to fall apart.
Next a Calormene, Emeth, one of his own men, decided to go into the stable. He went in, there was the sounds of a battle, and then a Calormene's body was thrown out. Rishda then told the party of talking animals gathered there that it was Emeth, but the Narnians hiding behind the stable (King Tirian, Jewel, Jill Pole, Eustace Scrubb, Farsight the eagle and Poggin) could see that the Calormene's body was a different person, and not Emeth.
When the Narnians jumped out and began to fight the Calormenes, Rishda called the reinforcements that he had.The battle was going in favor of the Calormenes, and no doubt Rishda would have eventually won, until King Tirian, who announced a fight, dueled with him and threw the stable door, unexpectedly sending them both into Aslan's Country.
Rishda was terrified when he saw Tash, who asked him: -
Rishda could not answer due to his great fear, and thus Tash picked him up, and then turned his attention to Tirian, but Peter told him to leave in the name of Aslan and the Emperor-beyond-the-Sea. Tash obeyed, possibly out of fear to both of the figures.
It is unknown what happened to Rishda afterwards. It may be presumed that as Rishda was considered a "lawful prey" by Peter, then he met the same fate as Shift - eaten by Tash, or worse, given Tash's similar nature to Satan. Furthermore, Tash is not at all pleased to be summoned to Narnia by Rishda.