You have two choices. 1. As both Daemons and Knights have the Chaos faction, then yes they can go in any together. But you lose some army special abilities for doing so (e.g. Contagions of Nurgle, Cabbalistic Rituals, etc.). 2. The *Fallen Hero* ability allows you to have a single *Dreadblade* unit (one Knight, or one War Dog squadron) in any army *without* losing special rules. (Though technically the Daemons Codex hasn't been updated yet to include rules that are lost if the entire unit isn't Daemons. But it will be eventually.)