The very presence of Grey Knights is nigh intolerable to Daemons. Their psychic purity and sonorous chants of detestation repel the daemonic and make it hard for them to even maintain their hold in the material universe. Daemons which take Instability tests while there are Grey Knights on the battlefield suffer a -1 modifier to their Leadership. All Daemons attempting to charge Grey Knights must roll dice for their Assault range as if they were moving in difficult ground.
Rites of Exorcism
Source: Codex: Daemon Hunters, p. 8