He is a Bishop in the Russian Orthodox Church. During the Third World War, he allies with Fiamma to capture Sasha for him in exchange that Nikolai becomes the next Arch-bishop of the Russian Orthodox Church. He's not the leader of Annihilatus, that position is taken by Vasilisa.
During the events of Volumes 9-10, when the 'St. Peter's Cross' was still thought to be the 'Stab Sword', Nikolai was the one thought to be on the recieving end of the deal for the artifact, due to his reputation of being ambitious and willing to use just about any method to get what he wants.