In each age, the infallible Lord appears in this world among the animals, human beings, sages and demigods. By His activities in these incarnations He protects the universe and kills the enemies of Vedic culture.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
रक्षाच्युतावतारेहा विश्वस्यानु युगे युगे । तिर्यङ्मर्त्यर्षिदेवेषु हन्यन्ते यैस्त्रयीद्विषः
rakṣācyutāvatārehā viśvasyānu yuge yuge | tiryaṅmartyarṣideveṣu hanyante yaistrayīdviṣaḥ
SB 12.7.14