Annihilatus

is the secret branch of the Russian Orthodox Church that deals with magic, specifically the disposal of spirits and such that shouldn't exist. To them, a wandering spirit should be eliminated, as it's 'something that shouldn't exist'. From volume 4, in Tsuchimikado's words, they are 'Ghost hunters' as opposed to the Necessarius being 'Witch Hunters'. Information about this organization is limited, with the only known members being Sasha Kruezhev and Vassilisa.