The presence of Grey Knights indicates intense daemonic activity and an imminent breach of catastrophic proportions. To represent this, Daemon Packs, Nurglings and Daemonic Beasts Packs (but not Greater Daemons, Daemon Princes or any other type of Daemon) gain the Sustained Attack scenario special rules (see the Warhammer 40,000 rulebook) in any mission where they are opposed by Grey Knights. Daemons brought back into play in this manner are not summoned (they re-enter the table from the edges as detailed in the Sustained Attack rules) and cannot claim mission objectives, table quarters, etc.
Designers' Note: The Daemonic Infestation rule is intended to offset some of the Grey Knights' considerable advantages when fighting Daemons. This is done rather than simply making them cost more points, so they are not penalised unfairly when fighting non-daemonic opposition.