Our next psychic disicpline is Divination and is already touted as one of the "must haves" of the psychic world. Only Space Wolves, Dark Angels, Blood Angels, Grey Knights and Eldar get this though between them they do have a lot of usefull Battle Brother Allies (basically Chaos armies, Orks, Necrons Sisters of Battle, Tyranids left out to dry). Let's see what the fuss is all about.
Primaris Power - Prescience. And this is where it really begins - a very decent primaris power. Not over the top but a solid choice. Any unit within 12" gets to re-roll all hits. Not just shooting but assault, too. This is Eldar's guide on Crack with double the range and double the impact - being able to affect shooting and assault. This obviously works very well on larger units or anything which expects to be in assault but a couple of psykers with this power isn't a bad thing at all and well worth their utility cost. Rune Priests in particular who have their still excellent psychic defenses could do worse than simply getting this primaris.
1 - Foreboding. Grants the psykers unit counter-attack and fires Overwatch at full BS (doesn't override inability to fire during Overwatch - i.e. blast weapons). This is a great spell for a bubble-wrap or leading unit or simply something you don't want charged. It's not a nice prospect running into even a full Tactical squad when it has an extra round of shooting and Counter-attack if you make it when you assault them. Probably not that great for actual counter-assault units unless you stick them out at the front of your army - otherwise the opponent is going to engage the rest of your army anyway. However, this does put the psyker at risk as it must target his squad.
2 - Forewarning. Friendly unit within 12" gets a 4++. Not great but not bad but particularly annoying with Grey Knights and Nemesis Force Swords (thanks for a 3++ in combat!). Any unit in the open, particularly ones which rely on their normally good armor values, aren't going to sneer at this. It removes the issues of cover 6th edition has brought to such units - i.e. individual models, general reduction to 5+, etc. and takes them back to a 5th edition surviability ratio but in the open. A large unit with poor saves as well could also benefit from this against shooting and in combat (i.e. blobs) as you are effectively improving their crappy save on the cheap. Either way, lots of uses can be found for a 4++ save you can move around onto differnet units - nice utility.
3 - Misfortune. The opposite of Eldar's Fortune and with a range of 24" - any enemy unit hit with this must re-roll all successful saves. That's Null Zone on crack against a single unit (are we sensing a theme here yet?) and is great for focusing firing big units off the table or helping deal with Deathstars. Paladins don't look so scary if they're hit with this (though remember AEGIS). This works particularly well combined with Eldar's Doom or any big units bringing all their firepower to bear (i.e. Grey Hunter units, Strike Squads, etc.) - you want to force as many wounds as possible. This power makes unit removal much easier and unless the opponent's unit is extremely durable or your firepower is crap, the unit is likely gone by the end of the shooting phase (though it's still in effect during combat). Excellent for any armies which can localise firepower well.
4 - Perfect Timing. Another power which benefits the Psyker and their unit - they now gain the Ignores Cover USR. This isn't as big a deal as it would use to be - cover was a lot easier to get and generally a 4+ in 5th edition whilst it's not so for 6th. That being said, it's still a nice utility option in relation to getting rid of units camping on objectives in area terrain or buffing their cover save through certain options (i.e. stealth, etc.). Obviously this is only really of use to a shooting unit as close combat ignores cover already.
5 - Precognition. Another blessing which targets the Psyker but only the Psyker this time. This allows the Psyker to re-roll failed To-Hit, To-Wound and saving throws. Nice for a combat psyker but there aren't a lot of those in the listed armies above. This is definietely a spell you'd like to change for Prescience if you get it as it's really not in tune with the rest of the psychic discipline. It's a nice option to have but generally not what you're looking for in terms of utility for a given psyker from the above armies.
6 - Scrier's Gaze. And another blessing which targets the Psyker only but this affects the entire army thankfully. When in effect, you can roll three dice and choose the result when rolling for reserves, outflank and mysterious terrain. This is huge when mysterious terrain is in effect as it actually gives you some control/relaibilty with what's happening. In terms of tournament play where that is unlikely to be used however, this generally has no effect. If however you have a lot of reserves or outflanking units, the triple dice gives you some excellent reliability in this regard but you'd need more than one or two units engaging in this activity to really get the most benefit from it.
Divination is best used with a shooting based army - so most of the good armies anyway. The best powers are going to get the most mileage out of shooting based units or constructs though obviously can add utility to combat oriented units as well. The basis of this all though is a good shooting element. The primaris power is what really makes this discipline shine - if you get one of the few bad powers or powers which don't really help your army in a given match-up, Prescience is a spell which you can always find use for; even with smaller units. Re-rolls are never a bad thing. Otherwise rolling on the Divination table is giving you a halfway decent chance of getting a semi-decent power. If you get it, great, use your second power to get Prescience and if you don't, you take Prescience and do something else with the other power. This is one of the few tables where the majority of Psykers can willingly say "let's trade." The major exception here is going to be Eldar Farseers though an argument can still be made for them as well.