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The complete Shloka is a as follows with its English translation
"Yadayada hi dharmansya glanirbhavti Bharatam
abhuytthanam adharamasya tadatmaanaam srajamyham
paritranayaye sadhunaam vinashya duskritam
dharmsanstha panarthya sambhavami yuge yuge..."
"When ever and where ever there is a decline in religious practice,O descendent of Bharta, and a predominant rise of irreligion-at that time I descend myself To deliver the pious and to annihilate the miscreants,as well as to re establish the principles of religion,I myself appear millennium after millennium ".