According to the Kalki Purana, the twin brothers Koka and Vikoka serve as generals under the demon Kali (not the Goddess), overlord of Kali Yuga.
These two brothers are supreme demons, great fanatics and adept in the art of war. They are intimately connected, powerful, hard to defeat and are even feared by the Gods. In their battle with Kalki, the 10th and final avatar of Lord Vishnu, the brothers display their mastery over the dark arts by raising themselves from the dead faster than Kalki can kill them.
The Kalki Purana states they are the grandsons of Shakuni and sons of Bakasura. These are not the same as the similarly named Shakuni and Bakasura from the Mahabarata. The former are father and son, whereas the latter are unrelated.
Lord Brahma eventually appears to Kalki during the battle and reveals to him that the twin devils had received from him a boon of "Immortality". No earthly or celestial weapon can kill them if they are allowed to hold onto one another. The only way for the dark duo to die is to separate and attack them both simultaneously. Sri Kalki then forces himself between the two and lands crushing blows to each demon’s temple at the same time. They both die, this time for ever.