Kalki Avatar is the future incarnation of Lord Vishnu, which is yet to arrive. Story of Kalki Avatar is mentioned in Puranas especially Srimad Bhagavad Purana and Kalki Purana.
Since the world has gone utterly corrupt with adharmic activities, it is believed the Kalki Avtar will bring an end to the present cycle of the universe and after a span of darkness creation will begin once again.
It is said in Bhagavad Purana that he will appear in Sambhala village.
Kalki is usually depicted riding a horse with a drawn sword. Lightning, thunder, heavy rain and harsh sun are frequently associated with this avatar.
As per the popular version of the story, the end of Kali Yuga will see even the pious neglecting the Lord. The sufferings of those follow Dharma will become unbearable.
Lord Vishnu will appear as Kalki as the son of Vishnuvyas in the village of Sambhala. Riding a white horse with a drawn sword, Kalki will mow down the evil and corrupt and ensure the reign of Dharma.