Hanuman Ji dressed up as a Brahman boy and started serving other Brahmans who was performing Yagna at Lanka. Seeing the unselfish service of Hanuman ji, Brahmans become pleased to him and asked him to take a boon from them.
Then Hanumanji said politely – Lord, if you are pleased with my service then the mantra with which you are doing Yagna, change one letter on my saying.
Brahmin could not understand the mystery behind it and said “Tthatsu”. Hanuman ji said, “In the Mantra of Jayadevi.........Bhutirharini, please chant “K” in place of “H”, it is my only wish”. Bhutirharini means who ends the sufferings of human being and after changing the letter H to K it becomes Bhutirkarini which means to give sufferings. The Goddess become angry with this and Ravan had to face defeat in the battle.
Hanumanji had changed the direction of Yagna by replacing only one letter in the Mantra.