Also check out Satorri's Guide: http://www.tankspot.com/forums/f218/...u-started.html
Thought I'd get this up to start sharing info. BIG NOTE: This is all speculative, but well researched. If I feel dodgy on something, I say so. Below is the cleanest strat/capture I could find on video. Its a 10man, but the encounter is identical and adding 15 extra folks wouldn't change it.
[ame="http://www.youtube.com/watch?v=91gSuvZfHIg&feature=PlayList&p=E9AA0787526 E694A&playnext=1&playnext_from=PL&index=89"]YouTube - Affenjungs INC vs. Valkyr Twins (PTR)[/ame]
Environment, Positioning, and Basics:
(1) UPDATE: The Dark Valkyr will start on the left side and the Light Valkyr will start on the right side, on the far side of your starting position. They will aggro when you run in range. This gives you time to get everything set up and for people to see what the portals look like.
(2) Split your raid into 2 groups of somewhat equal strengths. Have the group attacking the "Dark Valkyr" in the southeast and the "Light Valkyr" group in the northeast.
(3) Four portals will spawn. The "Dark Portals" will be in the northwest and northeast and the "Light Portals" will be in the southeast and northwest.
(4) On the pull, each group should click the OPPOSITE color portal of the Valkyr they are assigned to. The "Dark Valkyr" team should be "Light Attuned" while the "Light Valkyr" team should be "Dark Attuned." Your positioning should have the correct portal right on top of you to start. The reason for this is that Light Attuned players deal extra damage to the Dark Valkyr and absorb light damage. *UPDATE* Healers should be the *opposite color* of the team they are healing so they can absorb the same color orbs as the boss, if they are on a path to hit her (IE- "Dark Attuned" team[attacking the "Light Valkyr"] healers should be "Light Attuned").
(5) The Twin Valkyrs should be tanked near the SAME color portal on the far side of the room from your staring positions. This will come into play later.
(6) Light and Dark Orbs. There are orbs that spawn in a circle against the walls of the arena and fly, apparently in a random pattern, throughout the room. If a Light Attuned player "runs into" a Light Orb, they will gain stacks of "Powering Up." When 100stacks are reached, the player gains "Empowered Light," increasing damage done by 100% for 20sec. If a Light Attuned player collides with a Dark Orb, they take damage. I do not know how many charges each orb gives you. UPDATE: If you absorb damage from Vortex, you will also gain stacks of Powering Up, as well as splash damage from Unleashed Darkness/Light. *CONFIRMED* The boss gains stacks of Powering Up.
The catch is, the Valkyrs can also get this buff. Have one or two players designated for each Valkyr to "catch" the like color orbs before they reach the boss. In the video, the person doing this was in the opposite color. However, in 25man, it may be beneficial to have these designates in the same color as the orbs they are trying to absorb, as to not kill themselves trying to prevent them from getting to the boss. UPDATE: Then again, this may be bad since the range on Touch of Darkness/Light is unknown. 
Special thanks to Satorri for this graphic
Boss Abilities:
NOTE: I am only listing the Darkness abilities because they are identical to the Light ones. Just switch Light and Dark... you know the drill.
Empowered Darkness
Increases damage dealt to Light Targets by 100% for 20 sec.
Instant
This happens when the Boss absorbs too many Dark Orbs. Prevented by having designated Dark Orb "catchers" to prevent them from reaching the boss. It is my theory that, at least in 25man, these "catchers" should be Dark Attuned, even though they will be DPSing the Dark Valkyr. UPDATE: No stacks of Powering Up went up on the boss, according to our WoL. Either we were really good about the orbs never reaching the boss/tanks/melee, or this is only a player buff.
Surge of Darkness
*UPDATE*: Now on Normal Mode!
Pulsing a Surge of Darkness Inflicting 1500 damage every 2 sec to Light enemies.
5 sec cast
*UPDATE* This is a zone wide aura.
Unleashed Darkness
The Concentrated Darkness (i.e.-dark orb) releases its energies inflicting 12188 to 12812 Dark damage to enemies within 8 yards.
50000 yd range, Instant
UPDATE: It has been mentioned that this happens when an orb is touched, and I tend to concur. CONFIRMED! The splash damage from this ability contributes to "Powering Up" if absorbed by those with like attunement.
Twin Spike
The Twin Spike inflicts 100% weapon damage and increases damage taken by 20% for 15 sec or for 10 charges.
5 yd range, Next melee
*CONFIRMED* Spike damage ability on the tank. I do not know how often it is used, but it may be advized for your tanks to use a CD for the first time it happens to gauge how devastating it is.
Power of the Twins
The shared power of the Twins increases attack speed by 10% every second up to 150%, and allows the Twin to Dual Wield for 15 sec.
Instant
*CONFIRMED* *UPDATE* This occurs when the other Valkyr casts Shield of Darkness/Light. The Valkyr will gain 10% attack speed every second up to 150% or the opposite Valkyr's shield breaking. Just a damage spike on the tank. Might be another good time for a CD. Otherwise, tank healers have to be sharp to pick up this spike.
Touch of Darkness
<HEROIC ONLY>
The Val'kyr's Dark touch has affected you, inflicting 7313 to 7687 Dark damage to players under the effect of Light Essence every 1 sec. The Touch of Darkness will not harm targets with the same attributes as it.
50000 yd range, Instant, 15 sec cooldown
Ok. This is one of two abilities for why it is advisable to tank the Valkyrs next to the portal of the same color. If a Light Attuned player gets hit by this... simply move the few yards to the Dark Portal and click it to prevent further damage. I do not know if this wipes the debuff, or if the debuff has a duration (as none is listed). After it is cleaned, however, run to your starting position portal to become Light Attuned again.
This appears to be a mechanic to keep your "light and dark" groups separated. Its sort of like an XT light bomb for those who are Light Attuned. It is unspecified what the range or duration on this is, as well as whether or not it can be dispelled.
UPDATE: I'm still conflicted on how to deal with this ability. The description is a little confusing, as it says every 1 sec., but has no duration nor radius listed. If casted amongst the "Light Attuned" group (ie-attacking Dark Valkyr), it could be devastating. Here are the permeations:
(1) If it can be dispelled, dispel it ASAP.
(2) If its a small radius (ie-8-15yards) and has a duration, have that person run out of the group.
(3) If its a large radius and a short duration, (ie- 30yards, 5sec) it must be brute healed, while that person runs out.
(4) If its a large radius (ie-60yards-zonewide) and a long duration(10+sec), its best that everyone click the "Dark Portal" until the end of the duration, while that person runs away from the group.
It's my guess that its either (1) or (2), though I'm leaning towards (2). I'm also guessing it can be bubbled and ice blocked away.
Dark Vortex
Eydis Darkbane beings to cast a powerful vortex of Dark. Channels for 5 seconds inflicting 5850 to 6150 damage every 1 sec for 5 sec.
8 sec cast / Channeled, 15 sec cooldown
*CONFIRMED*This is the main ability where it is necessary to "switch" attunement. Note the 8second cast gives you plenty of time to get to the nearby Dark Portal (remember your tanking the boss right next to it) and click it to absorb all the damage. This ability is NOT interuptable. Once they have switched their attunement, the "Dark Valkyr" team must all move to the starting position light portal to re-attune. Do NOT re-attune until Dark Vortex has finished casting. Once re-attuned to Light, move back to the designated tanking position by the Dark Portal. *UPDATE* This ability can be ranged. Therefore, it would behoove healers to be Dark Attuned so they can absorb Dark Orbs that threaten to reach the boss without taking damage.
Shield of Darkness
A Shield of Darkness that protects the sister from harm absorbing 700k damage and prevents spell interruption for 15 sec.
Instant, 20 sec cooldown
This is the "signature" ability for this encounter. When Shield of Darkness is activated, the Dark Valkyr begins to cast...
Twin's Pact
The Twin's Pact heals for 20% of their total HP.
50000 yd range, 15 sec cast
You don't want this. Basically, you must DPS 700K in 15sec AND THEN INTERRUPT THE CAST to prevent this from happening. Again, just removing the shield doesn't stop the heal from happening. Now, on the 10man video, the "Light Valkyr" casts this first. You will see a big translucent bubble appear around her. Only a few of the Dark Valkyr Team dps switches over, and they didn't change their attunement for the increased damage. On 25man, this may not cut it. It is my recommendation that when Shield of Darkness goes up, all the ranged from the opposite team switch to the appropriate color attunement and assist in bringing down the shield. REMEMBER TO INTERRUPT TWIN'S PACT AFTER THE SHIELD GOES DOWN.
IMPORTANT NOTES:
(1) The Twin Valkyrs share a health pool. They must not "go down" at the same time. Think Kologarn, not Mimi.
(2) Healers need to be watching which abilities are being use to counter spike damage on the tank. Most of the raid damage seems to be avoidable, with a few single target spikes and a melee AoE spike.
(3) Absorb the orbs. Prevent same color orbs from reaching the Valkyrs, pick up same color orbs to build up stacks for Uber damage.
(4) I am uncertain if "Dark Damage" is the same as "Shadow Damage." At anyrate, doesn't hurt to have Prayer of Shadow Protection or Shadow Resist Aura up to test. My guess is its a completely different school. *NOTE*: It has been suggested, but unconfirmed, that "Dark Damage" is shadow and "Light Damage" is fire.
(5) *NEW* Every 45sec one of the Valkyrs will either cast Vortex or Shield.
Overall this fight seems to be about raid posistioning and technical execution. Seems like fun, can't wait!
As always, feel free to add info to this post that will further everyone's understanding of the fight. I'm also very certain that Prot Warriors will be very useful. =)
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