Lord Krishna to Arjuna (Shrimad Bhagavat Geeta)
“Whenever and wherever there is decline of dharma (righteousness) and ascendance of adharma (unrighteousness), at that time I manifest ‘Myself’ in visible form. For the protection of the righteous and destruction of the wicked, and for the sake of establishing dharma again, I incarnate Myself on earth ages after ages.”
Just like ‘the waves in an ocean,’ Lord Vishnu, the Supreme Soul and the sustainer, appeared down to the earth in dussa avataras or ten avatars. However, His Avatara (incarnation) as Krishna is considered as the God Himself incarnated on the eighth day of the new-moon fortnight (krishna-paksha) of the month of Shravana, when Lord Krishna appeared on this earth. Lord Krishna was born in the Duapara-Yug, which came just before the Kaali-Yug.