Kaliyuga is when evil is constantly rising. Virtue is on the decline. If we see what is happening around us, we would mostly agree that evil is rising. The world is not as it was in the past.
It is said that at the end of Kaliyuga, Lord Vishnu would come in the form of Kalki. This avatar will end the evil and the final war will end the world as we know it. Kalki will then lead whatever is left of the world to another cycle of Satya Yuga. Kalki will end Kaliyuga and start Sayta Yuga again.
In sacred texts, it is said that Kalki would be trained by Parashurama and other Chiranjeevi’s. They will help Kalki become the greatest warrior of all times. When his training will be done, Kalki would go seek guidance from Lord Shiva.
Lord Shiva would give him a sword, a horse and a parrot. The sword will help Kalki to end the war with evil. The horse would take Kalki anywhere in the world in an instant. And the parrot has all the knowledge of this world and will act as a guide for Kalki.
Equipped with everything, Kalki would have a final war with evil. This war would be unlike any other war. It will end the world. Anything and anyone that survives this war will follow Kalki as he sets up a new world and starts Satya Yuga again.
This avatar is a very interesting concept. This avatar acts as a placeholder for evils unknown. What’s even more interesting is to note that Kalki will receive the tools of destruction from Lord Shiva.
Destruction is an integral part of creation. New things can not be created unless broken things are destroyed. Lord Vishnu as the protector of the world knows that and so when he sees the world as too broken, he helps in its destruction so that a new world can emerge.