Since Rune Priests feature in every Space Wolves list I've ever written (or will write) I thought it was about time to discuss their powers. To start off with it's worth explaining that each army in 40K that uses psychic powers tends to have a particular style that matches the style of the army. For example, Eldar farseers have powers that benefit particular units in the army along with a couple of aggressive powers. The Blood Angels have powers that help in combat but very few with any range to them. So for Space Wolves as the happy-to-shoot-you-but-also-kick-your-teeth-in-if-necessary wing of the Adeptus Astartes we get mostly aggressive powers.
We're pretty spoilt for choice really as our rune priests can take two powers each at no extra cost. The difficulty is in deciding which combination of powers will work best. To start with here's the basics of the Space Wolves powers:
You move your priest into the middle of an enemy unit and put down a large blast marker on his head. Anything hit by it takes an S3 AP5 hit. The problem is pretty obvious in that your priest has to be right in the thick of it for this to work. Realistically against any decent foe it'll be the last thing he does so chances are you'll only get to use it once per turn. Even on a good day you aren't going to get more than a handful of hits and with it's stats you aren't going to kill much of anything thats T4+. Personally I'd avoid it. There are far better powers.
This power is the Daddy of rune priest powers. D6 unlimited range S7 lightning bolts with AP5. The range is the key thing here because it means there'll hardly ever be a time you can't get this power off. With a decent amount of hits it's good for demeching and can be nasty against Monstrous Creatures or other high toughness things. To use it on infantry is sort of a waste but OK if there's nothing else around.
Gives the rune priest and his unit a 5+ cover save. Since the new FAQ ruling that only one model in the unit has to have the stealth special rule for the whole unit to get it this power has suddenly found a use. Basically couple it with a Wolf Priest with Saga of the Hunter and that unit walks around with a 4+ cover save. Let's face it that's minimum 210pts of characters for one unit so it isn't really worth it. Not a bad secondary power but again there are better ones.
Any jetbikes, skimmers, jump infantry and deep strikers within 24” treat all terrain as difficult and dangerous. Initially this may sound pretty good but it's pretty situational. Can be handy for keeping your opponent off your objective, particularly in Capture & Control. Again a good secondary power that will serve you well against Blood Angels, Eldar and Daemons.
Fury of the Wolf Spirits
Shooting attack with 12” range and S5 AP- Assault 3 plus 12” S4 AP2 Assault 2. Additionally, any casualties mean morale check is required. It's a shame that this power doesn't give you better range or a few more hits. Again it's range means you're unlikely to get to use it twice. Only two S4 AP2 shots means you're going to be lucky to kill a marine or two with it. Again I'd avoid this one in preference of others.
Shooting attack with 18” range putting out an impressive 3D6 S3 AP- hits and unit treats all terrain as difficult and dangerous on next turn. This power seems to get overlooked by some and I think it's because people see the S3 and are put off. If you're lucky with your rolls you could put out upto 18 shots and even at S3 that translates to a couple of dead marines and a heap of dead orks/eldar/tau/guard etc. The real key to this power is battlefield control. Even if you don't kill anything in the initial volley you present your opponent with a choice. Stay put and waste a turn, perhaps only to be shot with it again or move and risk losing men to dangerous terrain (not to mention slowing them down significantly). Can be really effective against large squads and it's actually easier to kill marines with dangerous terrain than by shooting them!
Jaws of the World Wolf
Draw a 24" line and anything touched by it has to pass an initiative test or be removed! This can really swing a battle in your favour if you position yourself well. It's amazing how many things you can line up even with 24" range. Whilst you might be put off trying to use it against anything with initiative 4 and up think about this; how much pressure are you putting on essentially a 3+ save for a lot of characters not to perish. Even Ghazghkull will quite easily fall down the hole and be removed. What's more with the FAQ you're allowed to target specific models. Makes it easy to knock the teeth out of Ork Boyz mobs by targetting the power klaw nob.
Basically, if you're only fielding a single Rune Priest I'd recommend Living Lightning and Jaws. If you throw in a second priest then try Lightning and Hurricane. Storm Caller is a possibility if you feel yourself needing cover (particularly against shooty armies like IG) but really you should try to be on the offensive!
Stick your rune priest in a rhino and pop off Lightning from the hatch as you make your approach. Once you get in close you can unleash Jaws to weaken a unit before getting out and charging it next turn. Alternatively sit back with the long fangs and keep blasting off Lightning to help with transports. A nice little tactic I tried out recently against Matt's Orks was to put the priest and a squad of hunters in a drop pod then on turn one when everything is nice and neatly deployed you can line up a good chunk of the army. Unfortunately both the power klaw nobz i hit passed their test but a few boyz snuffed it at least.
With the second priest you can stick him on an objective with some grey hunters and use lightning up until a unit gets in range for hurricane. Keep them off the objective and pepper them with shots from the rest of the squad.
I realise I've not talked about Njal's Lord of Tempests but I've covered it a little in his guide.