When Kaikeyi was a child, a sage visited her father. The playful child, amused by his appearance, blackened his face. The sage was furious and cursed the child. He said she who had blackened his face would later acquire such a bad reputation that she would never be able to face anyone.
The child was frightened by the sage's words and tried to pacify him by giving him an iron rod that she had been playing with and told him that he might find it useful. The sage said that since she had been so considerate as to give him one of her playthings, he would bless her. She would be able to turn her hand into iron when she so wished. This blessing was of benefit to Kaikeyi later on after her marriage, and also resulted in Rama's exile to the forest, said M.V. Anantapadmanabhachariar, in a discourse.
Once when Dasaratha, Kaikeyi's husband, was fighting his enemies, one of the spokes in a chariot wheel broke. Kaikeyi then used the sage's blessing to help her husband. She turned her hand into iron rod, and propped up the wheel with it. Dasaratha was then able to fight and vanquish his enemies.
Pleased with Kaikeyi's help, Dasaratha said he would grant her two boons. Kaikeyi, however, did not ask him for anything in return. Years later, when Rama's coronation is about to take place, Mantara reminds Kaikeyi of the two boons her husband had granted her on the battlefield. Why should not Kaikeyi ask Dasaratha for the boons now, asks Mantara. Kaikeyi and Dasaratha had both forgotten all about the boons. It is Mantara who reminds Kaikeyi of what is her due and asks her to demand that Rama be banished.
Kaikeyi had been cursed by a sage, when she was a child. His curse comes true, for when Kaikeyi insists that Rama be banished, the people of Ayodhya and, even her own son, begin to hate her and revile her. It was the sage's boon that helped the curse to come true, for it was because of the curse that she was able to change her hand into iron rod and help her husband. It was her help to her husband that made him grant her the two boons that resulted in Rama's exile.