Choose between:
-2x Engine Guardian: Although the provided stat refill from this set is random, you will almost always be able to benefit from what you get. Great sustain set, you can basically solo bosses in some content with the resources given by this.
-2x Lord Warden: Strong set for when you stack constantly with your damage dealers (vAA suits this set nicely). Gives them increased resistances and allows you to hit cap aswell.
-2x Sentinel of Rkugamz: Magicka Nightblade tanks do a fairly large amount of healing over time, allowing them to make good use of this set. If you're running with Stamina Damage Dealers, this set benefits them greatly, but generally, so does Lord Warden.
-2x Swarm Mother: If you enter a trial or dungeon which has a fight you will need a form of DK chains for (vMoL is a big one, Mazzatun, Cradle of Shadows, ICP aswell), then this set is the go to. It also allows you to pull in those pesky archers and mages in the back line placing them in your team dots more often and giving your team a faster clear time.

Choose between:
-5x Tavas: Works with Mirage and Roll Dodging to generate large amounts of ulti quickly for frequent Warhorns.
-5x Ebon Armory: Good utility set that benefits the whole team if they need a little more survivability.
-5x Lunar Bastion: Another solid utility set, gives your team a shield that refreshes often. It may be a small shield but its nothing to sneeze at when stacked with other ones.
-5x Rattlecage: If you are tanky enough and need to deal more damage (dungeon assassin challenges in most cases) use this, as it gives you a nice amount of damage while giving you a constant source of major sorcery.
-5x Seducer: Nice starting set for Magicka Nightblade tanks, and good for pug runs where your team might take a while to kill bosses.

Choose Between:
-5x Alteration Mastery: If you want to not run out of resources this helps a lot. Same with Seducer great to start with and good for pug runs.
-5x Mending: I want to test this set to see if it procs from Refreshing Path, if it does holy moly you can do good things with this set, it affects all damage bosses do, not just weapon damage. Its basically sort of a major maim debuff for things around you.
-5x Worm Cult: Helpful when running with a group of Magicka builds. The healer would probably run this but if he doesn't mind go ahead and use this for the sweet cost reduction.
-5x Dragonguard: Reduced ulti cost, more frequent Warhorns. Don't use this in trials, your team will usually have a good enough Warhorn uptime if the healers and tanks have the skill.
-5x Alkosh: While not great for Magicka, the 2.7-3k resist debuff is too good to pass up. Try to get synergies when facing the boss as often as possible, things die fast when this is applied.