10 Normal strategy
Also known as "double dragon", this is a fight versus a purple and blue dragon with a shared health pool. They begin with Variona on the ground, and Theralion in the air, and switch every 25% hp, gaining a different set of abilities in the process. Theralion's ground phase is by far the harder of the two, since Variona deep breaths constantly throughout.
A couple of boss abilities may cause you to get warped into the "twilight zone", a special phase where you take constant, stacking dot damage until you escape in a portal. It is very hard to survive this place, so try not to get hit by anything that you aren't supposed to.
Blackout - One random player is tagged with a dispellable debuff that absorbs healing. Once the debuff expires, whether by dispel or healing, the affected player explodes, dealing a large amount of shadow damage split between nearby targets, meteor style.
Devouring Flames - Valiona fills half the room with flames. Just run behind her.
Dazzling Destruction - Theralion, who is flying, casts these huge purple swirly whirlpools on the ground. If you stand in them, you get ported into the "twilight zone".
Twilight Blast - A simple shadow-based fireball that targets one raid member and blows up, hitting them and anyone nearby for moderate shadow damage.
You only need one tank for this encounter. Simply run through her if she starts to breathe in your direction. Make sure you run out of the purple swirlies on the ground.
Spread out, but stay near enough that you can run to the other side of the room when she breathes. When Blackout is cast, stack up immediately, and spread back out once it expires. Don't stand in the purple swirlies, and stay away from the stairs, since they can spawn there, but the graphic is hidden.
Deal with the hazards the same way, but make sure you don't heal the Blackout victim until everyone is grouped up. Otherwise, healing is not that difficult during this phase.
Engulfing Magic - A horrible debuff that makes the target explode every time they deal damage or healing, including hots and dots. The damage will hit all nearby raid members, so the affected player must run out of the group.
Fabulous Flames - A very large void zone that ticks for minor shadow damage.
Twilight Meteorite - One unlucky target gets hit with a classic meteor attack, splitting damage between all nearby raid members.
Deep Breath - Valiona, who is flying during this phase, will fire an Onyxia-style deep breath covering a straight line in front of her.
Try to pull him out of the void zones, though they are large, and watch out for deep breaths. Make sure you don't accidentally drag the raid into one, because it is an instant wipe. You can angle your camera to see where Valiona is pointing if necessary.
Bunch up on the dragon's tail. If you get engulfing magic, immediately run out of the group. Watch out for deep breaths, as it is very easy and common to accidentally stumble into one while running out. Strafe out of the void zone when it drops to make sure you don't keep standing in it. Attack the dragon from the side if you must; the void zone is huge and hard to avoid. You will be moving constantly in this phase.
Raid healing is very tough here. Otherwise, all the same rules apply to you as the DPS. Assume that the engulfing magic will hit people, and start casting your favorite AOE heal the instant you see the spell warning.