The tenth incarnation of Vishnu. As per Agnipurana Chapter 16, by the end of Kaliyuga, everyone would be atheists and people would have no value systems. They will practise adharma in the guise of dharma. People will become cannibalistic. At this time, Vishnu will be born as Kalki, son of Yashas, and as a Yajnavalkya scholar. He will use his knowledge of weaponry to annihilate evil doers, and reestablish the four varnas. He will then give up the kalki form and ascend swarga. Once again, a Kritayuga would begin.
From Bhashabharata Chapter 190, once Kaliyuga is at its apex, Vishnu will incarnate as Vishnuyashas in a village named Shambhala. He will then become famous as Kalki. He will be a purohit of Yajnavalkya. He will be able to create armies and weapons with only his thought and will vanquish his enemies. Then Kritayuga will begin again. There is a difference of opinion on whether the incarnation has already happened. But going by the calculations in Mahabharata Vanaparva, it will happen only after another 4 lakh plus years.
As per Bhashabharata Chapter 190,