The first Dussehra was celebrated on Earth when the nectar in Ravana’s body was dried up by a weapon from Sri Rama’s quiver and Ravana, the symbol of unrighteousness, died after being hit by another divine weapon. As soon as the evil king fell on the ground, an extremely bright divine light soon emerged from his body and entered Rama’s own body, according to the Adhyatma Ramayana. For the spiritual, this moment was not less significant than Ravana’s getting killed, for it showed Rama’s acceptance of Ravana’s soul, which was also a fragment of the Supreme Soul.
This moment showed the world that Bhagavan Rama did not really fight a battle with an evil jiva on that Dussehra. Rama, who is beyond all karma and nature, had only relieved Ravana from his tamoguni prakriti (dark nature) and his previous bad karma. Rama’s sharp weapons did not destroy Ravana, the individual being, but conquered the root cause of his negativity* — the tamas qualities he had gathered over a lifetime or more. Why did Ravana need Brahman-incarnate Rama to cleanse his karma? We could say that the amount of tamas (coupled with power) accumulated by Ravana was so high that a major adjustment to the world was needed. Not only Ravana but beings who were suffering because of his bad karma, including saints, gods, planets, and the citizens of Lanka, also had to be freed from the influence of his ignorance. And nothing is more liberating than the darshan of Rama.
*It would be worth revising that no being is everlastingly evil in Hinduism. Being good or bad is a consequence of the functioning of the three modes of nature.