Once Ravana was performing Chandi Paath. Hanumana disguised himself as a Brahmin boy and started serving Brahmins performing Yajna. Pleased with his devotion they asked him to put forth one wish of him which they promised to fulfill. Hanumana said in a humble tone that all I want is them to change one letter of the hymn they were chanting. Brahmins didn’t get him, but said Tathastu (so it be) anyway. Now Hanumana categorically told them that he would want to change the letter ‘ha’ to ‘ka’ in a word called ‘bhurtiharini.’ Thus the “Bhurtiharini” became “Bhurthkarini.” “Bhurtiharini” means one who helps us getting rid of trouble and “Bhurthkarini” means one who creates troubles in the first place. Thus a supposedly adulation for Chandi devi turned upside down by merely changing one word. This enraged goddess beyond measures and she cursed Ravana. The Chandi Paath yajna to seek blessings of devi turned out to be a spectacular failure.
2) When Rama was willing to offer his eyes to Chandi
During the war with Ravana, Lord Brahma had advised Rama to worship Chandi Devi to gain strength to assassinate Ravana. Following the advise Rama arranged 108 neelkamal (blue lotus) to worship the goddess as per rituals. On the other side, to attain immortality Ravana too started Chandi Paath. Indra dev asked Pavan Dev to alert Rama about Ravana’s chanting of Chandi Paath and start the worship without much ado. Suddenly, one neelkamal went missing from the offering. It was due to a magic spell cast by Ravana. Now Rama’s determination was at stake as chances of Chandi Devi getting enraged were rife. Rare neelkamal could not be arranged immediately. One thing struck Lord Rama’s mind : People call him Kamal Nayan, the one whose eyes resemble Lotus. So he decided to sacrifice one of his eyes for the offering. The moment he pointed his arrow at his eyes, Devi Chandi appeared and stopped him. She blessed him to emerge victorious in all battles.”
3) When devas created Durga
In an another and most famous legend concerning Navratri, devi Durga slaughtered a buffalo-bull asura called Mahishasura. Devas had blessed him to be invincible after his rigorous pursuits, but once getting the power he started misusing it. He went on to become the lord of Swarga loka and devas had to exile to earth. Together they created Durga and reposed all their powers in her. They even provided her with their arsenal. A terrible fight ensued and kept on going for nine days until Durga emerged victorious and came to be called as Mahishasura-mardini or the one who slaughtered Mahishasura. To mark that victory, Navratri is being celebrated for nine days.