According to the legend, Lord Rama had seen Neelkanth before and after killing Ravana, due to which seeing this bird on the day of Dussehra is considered a symbol of victory. At the same time, by killing Ravana, Lord Rama was committed by the sin of killing a Brahmin. Shiva was pleased and took the form of Neelkanth and gave darshan to Ram ji, due to which he got freedom from Brahmin murder defect.
