During the battle of Mahabharta Lord Krishna said this shloka to Arjuna
“Yada Yada Hi Dharmasya Glanirva Bhavathi Bharatha,
Abhyuthanam Adharmaysya Tadatmanam Srijami Aham’.
Whenever there is decay
of righteousness O! Bharatha
And a rise of unrighteousness
then I manifest Myself
Praritranaya Sadhunam Vinashaya Cha Dushkritam
Dharamasansthapnaya Sambhavami Yuge-Yuge.”
for the protection of the good,
for the destruction of the wicked and
for the establishment of righteousness,
I am born in every age.