Cult of Magic
"Dedicated to the pure and unfettered use of sorcery."
This is a 9" smite / Infernal Gateway type power requiring the closest unit, but causing D3 mortal wounds on the "UNIT" (not model) and any units within 3" suffer an additional wound.
When your Warlord inflicts a Mortal Wound with a Psychic Power, add 1 additional Mortal Wound to the total.
Add 1 to your Psychic tests.
The implications of this power are obvious. Any mutli target, or splash type of damage power becomes that much more powerful with this Warlord Trait. The fact the Cult comes with its own splash damage power is an easy but effective combo.
Note that Astral Blast is still quite short at 15" with the Thousand Sons Legion Trait. This may require an extra support power supplied by another sorcerer. For me that's been "Warp Time". Bouncing your Warlord into danger range, and pulling him back, is pretty critical until you can properly create a buffer between him and your opponent.
The other power that comes to mind with such an ability is Infernal Gateway. Naturally this is available to your Daemon Princes and is also a power that causes D3 (potentially more on a 12+ test) and then additionally every unit within 3" suffers another D3 (+1) thanks to your Trait.
There are tools to allow you to add even more to your psychic test results giving a stronger chance of a 'super smite' causing D6 plus 1 Mortal wound to the target. (I have in fact done 7 mortal wounds to a Vindicator with 7 wounds left... it's a long shot, but quite satisfying at the right time!)
So what are you favourite combo's with Magic? Do you take a larger detachment with this ability? Or a simple detachment?