Abilities:

General:

• Dark Essence - Absorbs Dark damage, reduces damage done against Dark targets, and increases damage done against Light targets. Absorbing Dark damage can proc Surge of Speed. Gained by clicking a Dark Essence.
• Light Essence - Absorbs Light damage, reduces damage done against Light targets, and increases damage done against Dark targets. Absorbing Light damage can proc Surge of Speed. Gained by clicking a Light Essence.
• Surge of Speed - Increases movement speed by 70% for 15 seconds.
• Concentrated Light/Darkness - Floats around the room. When a player or NPC of the same Essence absorbs it, they gain Powering Up. When a player or NPC of the opposite essence absorbs it, they take damage.
• Powering Up - Causes Empowered Darkness or Empowered Light at 100 stacks.

  •  Empowered Darkness - Increases damage done against Light targets by 100%. Restores 20% of the target's maximum mana. Lasts 20 seconds.
  • Empowered Light - Increases damage done against Dark targets by 100%. Restores 20% of the target's maximum mana. Lasts 15 seconds.
• Unleashed Dark - Inflicts 7,800 to 8,200 (25-Players: 8,775 to 9,225) Dark damage to enemies within 6 yards. Used whenever a Light player touches Concentrated Darkness.
• Unleashed Light - Inflicts 7,800 to 8,200 (25-Players: 8,775 to 9,225) Light damage to enemies within 6 yards. Used whenever a Dark player touches Concentrated Light
• Twin Spike - Inflicts 100% weapon damage and increases all damage taken by the target by 20%. Each hit depletes a charge from the damage debuff. Lasts for 15 seconds or 10 charges.
• Shield of Lights / Darkness - Absorbs 175,000 (25-Players: 700,000) damage and prevents spell interruption. Lasts 15 seconds. Cast immediately before Twin's Pact.
• Twin's Pact - Heals the Twin Val'kyr for 20% of their total health. Cast immediately after Shield of Lights / Darkness. The 15-second cast can be interrupted if the shield is down.

• Power of the Twins - Increases Physical damage done by 20%, hit chance by 20%, and attack speed by 10% for 15 seconds (Val'kyr Quickness). Used only if Fjola is less than 60 (?) yards from Eydis. Both Twins gain this buff.
• Val'kyr Quickness -  The Power of the Twins increases attack speed by 10% for 15 sec. Stacks.

Fjolja Lightbane:

• Light Essence - Fjolja Lightbane is a Light creature. She takes less damage from targets with Light Essence and more damage from targets with Dark Essence. Her Vortex does Light damage.
• Light Vortex - Inflicts 5,850 to 6,150 (25-Players: 8,775 to 9,225) Light damage every 1 second for 5 seconds. Channeled after 8-seconds cast. Affects only targets with Dark Essence.
• Surge of Darkness -  Pulsing a Surge of Darkness Inflicting 2500 damage every 2 sec to Light enemies.
  

Edyis Darkbane:

• Dark Essence - Edyis Darkbane is a Dark creature. She takes less damage from targets with Dark Essence and more damage from targets with Light Essence. Her Vortex does Dark damage.
• Dark Vortex - Inflicts 5,850 to 6,150 (25-Players: 9,263 to 9,737) Light damage every 1 second for 5 seconds. Channeled after 8-seconds cast. Affects only targets with Light Essence.
• Surge of Light -  Pulsing a Surge of Light Inflicting 2500 damage every 2 sec to Dark enemies.

The Fight:

Twin bosses, shared HP pool, opposite aspects, Light and Dark.   When you talk to the NPC to get them out, they're be 4 portals that appear.    Two on each side, 2 of each other.    Looking in, closest left is Light, farthest left is Dark, also where the Dark Twin will be.     Closest right is Dark, farthest right is Light, also where the Light Twin will be.   Clicking on the portal will gain you their respective buff.

Split the raid in half.   Each side basically taking the portal closest to them to attack the twin on their side.  You must attack the Twin of the opposite color you gain.    You'll not only do more damage to the Twin of the opposite color, each twin also pulses an aoe that hurts ones with the same color as themselves, while the same colors will just absorb the damage   Though, I believe it's totall healable in the Normal versions.

Note:  A tip for the warlocks before the fight starts, if you all click on the Light Portal and spam Hellfire, you would start gaining stacks.   This way you can start ahead of the rest of the raid and get Empowered Darkness/Light much earlier.   Just switch back to your assigned portal before you start, the stacks do carry over regardless of color.

Anyways, after you start, there are 3 things that can happen.

First, Twin Pact.   One of them will cast a shield that absorbs damage, you must burn through it in 15 secs and interrupt the heal that comes after.    In Normal, you probably have enough dps to just handle each side on their own.     Be wary on Heroic this may not be the case and the guys on the other side must jump in to help, perhaps switching colors to boost dps.    Switch back afterwards.

Second, Vortex.    One of them will start to channel a vortex which will do massive aoe to everyone of the opposite color.    So whichever twin that's channelling, the raid must switch to that color to negate all the damage.    That is, if Dark Twin casts Vortex, everyone switch to Dark portals.    If Light Twin casts Vortex, everyone switch to Light portals.    Switch back to default colors afterwards.

Third, Concentrated Light/Darkness.    Floating orbs will spawn out and randomly move all over the place, they'll be either light or dark ones.    "Bullets" if you want to call them that.     Your goal is to avoid the ones of your opposite color, they'll just explode on you and hurt you.    And to absorb the bullets of the same color,  Light on Light, Dark on Dark.    When you do, your absorbed buff goes up.

That's another thing, you get a Powering Up debuff, and as you absorb more the stacks go up, to 100.  Once there, you get a 15s 100% damage boost for moar dps.

That's about it, shared hp pool so split raid accordingly and burn them down.

Have fun, far easier fight in normal mode than we expected.

Hard Mode:

General:

• Surge of Speed - Increases movement speed by 70% for 10 seconds.
• Unleashed Dark - Inflicts 7,800 to 8,200 (25-Players: 14,625 to 15,375) Dark damage to enemies within 6 yards. Used whenever a Light player touches Concentrated Darkness.
• Unleashed Light - Inflicts 7,800 to 8,200 (25-Players: 14,625 to 15,375) Light damage to enemies within 6 yards. Used whenever a Dark player touches Concentrated Light
• Twin Spike - Inflicts 125% weapon damage and increases all damage taken by the target by 20%. Each hit depletes a charge from the damage debuff. Lasts for 15 seconds or 10 charges.
• Shield of Lights / Darkness - Absorbs 300,000 (25-Players: 1,200,000) damage and prevents spell interruption. Lasts 15 seconds. Cast immediately before Twin's Pact.

Fjolja Lightbane:

• Light Vortex - Inflicts 6,825 to 7,175 (25-Players: 12,625 to 13,325) Light damage every 1 second for 5 seconds. Channeled after 8-seconds cast. Affects only targets with Dark Essence.
• Surge of Darkness -  Pulsing a Surge of Darkness Inflicting 4500 damage every 2 sec to Light enemies.
• Touch of Darkness - The Val'kyr's Dark touch has affected you, inflicting 2925 to 3075 (25-Players: 5,850 to 6,150) 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.
  

Edyis Darkbane:

• Dark Vortex - Inflicts 6,825 to 7,175 (25-Players: 12,625 to 13,325) Light damage every 1 second for 5 seconds. Channeled after 8-seconds cast. Affects only targets with Light Essence.
• Surge of Light -  Pulsing a Surge of Light Inflicting 4500 damage every 2 sec to Dark enemies.
• Touch of Light -  The Val'kyr's Light touch has affected you, inflicting 2925 to 3075 (25-Players: 5,850 to 6,150) Light damage to players under the effect of Dark Essence every 1 sec.  The Touch of Light will not harm targets with the same attributes as it.

Ok so everything hits harder, that's a given.   The Shields will take more damage to pop, so you want to tank the twins more on the insides of the portals, closer together so it's easier to switch

This Touch of Darkness/Light thing is a new trick, kind of a you are the bomb and start aoe'ing the guys around you.     The way to get rid of it is to click the portal of the same color.    Light goes to light.   Dark goes to dark.   And I believe it'll only target opposite colors so you have to switch, Touch of Darkness of Light buffed guys, and vice versa.     Ice Block, Cloak, Bubble, etc of course can remove the buff for you.     Anyways, switch back to your color if you have to once you got rid of the debuff.

The Concentrated Darkness/Light do hit harder, so more emphasis on not failing to them and hitting the right ones while avoiding the wrong ones.

Otherwise, same fight as always, just more tightly tuned to be a gear check again....

Hard Mode Soaker Strat:

Well because of the sheer amount of damage the orbs do cause, this is a method to "cheese" the fight.    It basically will take most of the skill in dodging orbs out, and replace with RNG.

1-2 Tanks, 1 tank can do it, just mind threat issues.   And if your dps is behind, you probably need that extra dps from the tank spot.

6 Healers ish, need them to keep every group alive from the damage and tanks and soakers.

2-3 Soakers, these guys are to get max SR, 415 buffed, and take the Light Essence, and go around taking up all the white orbs.   The SR is so that they don't have to continuely avoid the black orbs to get to white orbs, and just resist that damage.   Well most of it, you can probably survive only 2 orbs exploding on you at once, don't risk the 3rd.     Best type of soakers are the ones that have a mitigation CD, hunters especially since they can Deterrance.

Anyways, the raid is to take up the Dark Essence and tank both twins stacked together at the gates.  We opted to have the tanks outwards, and the dps/healers against the gates, this reduced stray White Orbs from exploding in the raid for massive raid damage.   This reduced the strain on the healers.    Though you can do the reverse, with tanks against the gate and dps outside, that will likely increase dps from Empowered Darkness, but risk those White Orb explosions the soakers didn't get to.     You do not move or change colors for the entire fight, you heal though all the extra damage and even dps with the wrong colored shield if you have to.

The entire fight you all pretty much dps the Light Twin, kill her down ASAP.   Except, when a special ablility appears.   1 of 4 special abilties will occur each time, random order, but they must be cycled through before it randomized again.     And it is possible to get 2 of the same abilties in a row for the 4th and 5th ability.    Anyways, here are the abilties and what must be done...

Dark Shield - This is potentially the hardest part of the fight, well for your dps.   We opted for that if this was the first special ability, we ignore it and let it heal and dps on.   Hoping, that rng favors us, or we have high enough dps to kill them before the next Dark Shield appears.    

If it not the first ability then you can't ignore it.   You gotta prep, like 5 seconds before every special ability CD (except the first) to switch to the dark twin, get your dps debuffs on her, and pop heroism and all CDs.    You will probably wipe if you don't get the kick off in time.

Light Vortex - Since the raid is all Dark and won't be changing, this will cause massive raid damage and it's now the healer's hardest part of the fight.   Assign your aoe healers keeping groups up.   Assign pallies to Divine Sac and Aura Mastery Fire Aura.   Everyone should be popping healthstones to help.    Also, if you got damage mitigation CDs, use them.   Your goal is to survive.   If people die, you'll probably wipe.    Also, RNG again, if a 2nd Light Vortex happens, you'll probably wipe.

Dark Vortex - This will merely power up most of the raid.  The soakers, this is where they have to pop their defensive CDs to survive it.   Hunters Deterrance, etc.   Healers pay attention to them in case.

Light Shield - We should all be on her already doing high dps with the buffs, should burn it easy and kick it.

That's about it for the fight, dps zerg it down.   Hope you don't get bad abiltiies close together, and kill them before they kill you.

Achievements:

Salt and Pepper - Defeat the Twin Val'kyr in 3 minutes or less in 10/25-player mode.

Seems to be just a dps burn fest.   Utilitize those buffs you can get.

Videos:

10 Man (w/ Salt and Pepper on our first shot)