Daemonology - Malefic:
Hell yeah, here we go:
Primaris: Obviously awesome, given the huge range of units that can be summoned. Whatever you're short on, you can make more. Of course, it isn't easy to pull this power off at ML3. I still can't shake the feeling that ML3 powers aren't going to be pulled off successfully very often. I think that's the idea, and I guess I can appreciate it when it comes to conjuring an entirely new unit you didn't "pay" for. At any rate, this power is purely for the greater daemons, daemon princes, and sorcerers upgraded to max warp charges.
Cursed Earth: Fucking awesome, especially because it's ML1. This will allow a unit of Horrors to buff the entire back field, freeing up the Grimoire for more mobile units. Only need to have 1 model from the unit within 12". Flying Daemon Princes could potentially facilitate a nasty deep strike that also protects the units who arrive. Unfortunately, ML1 is a blessing and a curse in terms of being able to pull it off and also dispel the power successfully. At any rate, the power itself is still good and very useful if you pull it off.
Dark Flame: Great, I love it. This is what the primaris power from Pyromancy should have been. Why is this power not in Pyromancy?
Infernal Gaze: At first glance this sounds good, and I guess for 1 charge on a flying nukeprince it wouldn't be bad. But it's a beam at strength 3, so you're hitting 3 units max. I think fleshbane is far more useful than armourbane on this ability. Practically guaranteed 3 wounds; on a unit of monstrous creatures with bad saves that could be very nice. 18" is great range, too.
Sacrifice: Sounds pretty amazing. "Options" says to me that loci or rewards are both fair game. Grimoire, Portal Glyph, loci... awesome! 1 warp charge and a pawn for a herald? Truly evil.
Incursion: Not sure I'd ever use this power over the primaris/possession but it's still awesome.
Possession: Whaaaa! This is the one they're gonna deny. As others have been saying, saving this for when you have a single wound left sounds like a decent way of using it. However, the likelihood of perils is pretty high, so you might just lose that wound and get nothing out of it. That's the way it goes, randomness and dice and all that.
What I like most about Daemonology is that you can run it as a nuke/buff discipline. If i'm correct, you can't get a 3 warp charge power unless you're ML3. At least that's the way it would have worked in 6th. Therefore, spamming ML2 psykers would allow you re-roll the powers that you can't cast. That means witchfires/cursed ground/sacrifice are guaranteed over the powers you can't use. Again, that's assuming this works the same way it did in 6th, which it probably doesn't.