The fourth and second-last fight in Trial of the Crusader is the Twin Val'kyr. The Twin Eredar are magic-based mobs who love to buff you with their orbs. :)
Pull: One needs two tanks. You can tank them seperately, but there's no reason not to tank them on top of each other. They don't cleave or anything. And all of your raid's cleave effects will hit both Twins. Just mark them both to assist with target switching. As DK, park behind your selected Twin and bang away.
Icy Touch while running in. Plague Strike. Regular dps rotation blah blah (sound familiar? :P). Be sure to pestilence early to get diseases on the second Twin. Whether you're rolling diseases or a more traditional spec, I'd recommend pestilence once every disease duration to ensure the extra damage - you do know they share a health pool, right? So any damage to one is damage to the other. It's definitely worth doing for every spec but non-GOD (glyph of disease) blood. For non-disease blood experiment, since pestilence will mean you lose a heart strike - I'm not sure which is better. For frost, unholy, and GOD blood (or 4pc T9), definitely pestilence!
DON'T pop your cooldowns at the height of your stacked buffs right at the beginning this time! Normally, you'd use your ghoul/gargoyle/dancing rune weapon when your ap/strength is highest at the beginning but you want to save them for Twin Pact. Throughout the fight, when orbs spawn, pop your anti-magic shield as they approach you. This ability is uber in this fight! It protects you from being killed by orbs, and it gives you a huge boost in runic power - be prepared to spam death coil while your shield is up!
Be careful throughout the fight. The biggest risk of dying here is ignoring the orbs and getting too many of the opposite color from you and killing yourself.
Twin Pact: I usually recommend the raid NOT heroism the first Twin Pact. Burn your cooldowns here! Switch fast to the Twin with the pact, and put everything into dpsing the shield down, except for enough runic power for mind freeze. Sure, there's lots of classes capable of interrupting, but there's no sense in missing the interrupt and leading to a wipe. Our melee compete to see who gets it and we give a gold to the winner. Heroism/bloodlust the second Twin Pact so that you can get it down in time. IF you have a third Twin Pact - and I really hope you don't, then your cooldowns should be back up to burn down the shield. Tiny side note here is if you're rolling diseases, switch targets and immediately pestilence to ensure both the 'new' target and the 'old' have fresh diseases. When you go back to your normal target, pest again just to keep your stack up. Also, wowwiki suggests switching color to dps the opposite Twin. DO NOT do this - you'll spend your time running instead of dpsing.
Vortex: Pay attention here! More than once I've started to run to change my color when I was already the right one. Of course, you do need to run to change if the Twin casting is the opposite color. Deadly Boss Mod is excellent here - it will flash a warning to switch color! It's not super fast, so you have a couple seconds - but don't miss! DK (rolling diseases or not) should pestilence to refresh diseases, run to the portal, click it, run to the Twins, pestilence again, run across to your normal colored portal, and run back to the Twins, and pestilence once more. You'll never lose diseases the whole time whether you're GOD spec or not. Running back and forth is a good opportunity to grab a few appropriately colored orbs, but be careful to not get greedy and run through opposite orbs as your healers are moving too and you can die easily.
Heroic: If you're on heroic, the fight is basically the same, but the health of the Twins is larger and the time you have to get the Pact shield down is much shorter. Twins will heal for more, so it is imperative that you switch if necessary and go nuts to get the shield down. You can't be as nonchalant about dodging opposite color orbs as they hit hard on heroic and will kill you, so keep an eye out especially if your anti-magic shield is down. I've survived not switching during vortex on normal difficulty with ams and icebound fortitude, but you won't survive it on heroic.
Your dps should be very competitive on this fight as well. You'll get cleave damage if you're blood, and tons of disease damage if you're unholy. If your frost, howling blast will hit both. On 25 man, I'm very disappointed in myself if I'm under 10k dps.