Howdy Guys and Girls :)
Another cool part from GSIII - Daemonic Adversaries. Remember the little fluffy rules we had back with the old Daemonhunters Codex? Well, they are back now and called "Daemonic Adversaries"!
The rules can be used in any battle that includes atleast one unit with the Grey Knights faction, if both players agree. They are there to give you a reason to field Grey Knights against Xenos or other stuff in fluffy battles :)
One option is the Daemonic Possession - an upgrade for your Warlord. This gives him the daemon, fleshbane, armourbane and smash special rules, aswell as a 3+ invulnerable save.
If the model is NOT from the following factions: Genestealer Cults, Necrons, Tau or Tyranids; any independed character can also be upgraded to a Psyker with Mastery Level 1/2 (and the daemons special rule).
You can also upgrade any psykers in your army with the daemon special rule (except those with the Genestealer Cults or Tyranids faction).
Last but not least you can get some Chaos Daemons allies.
This rule is quite similiar to GSI's warpstorm table, but it's more for buffing the units in your army. Here's an example for something really nasty that can happen ;)