The Crimson Hall (Cause it's full of red blood... get it?)
To get to the Plagueworks from the upper spire portal, you take a left. Blood wing is straight across the bridge, past the duo of val'kyr. They're easy targets, just have your tank pull them one at a time and throw out some AOE for the Severed Essences they copy from the players in the raid. Keep in mind that copies of priests and paladins will heal the val'kyr and the other essences as well as themselves. They should be first priority, although with the buff they're not as scary as they used to be. Once these two winged ladies are down, head across the bridge to the far side of the room where you'll encounter the first sets of blood trash. In these trash pulls, you have a number of gimicks to worry about depending on which Darkfallen you're fighting. They come in groups and your tanks need to be ready to pick them up as a mistanked mob can cause a wipe. The Advisors (which look like liches) do an ability called Lich Slap that knocks the target back. They also have a bubble like ability that protects one of the other npcs from either physical or magic damage. The Archmages polymorph you into a spider, which can be cleansed. The Blood Knights use Blood Mirror and Vampiric Aura to redirect damage and heal themselves. Commanders like to charge and knock back. Lieutenants like to throw out a curse that leaves a DOT on you when you're healed. Nobles enjoy using a banish on your raid members. And lastly, Tacticians sap your tanks and raid members, leaving them stunned for several seconds. These guys come in a number of combinations in 4 groups before you hit the Princes, one in the hallway outside their chamber, and the last 3 inside the chamber near the door. The last set is actually a patrol that walks back and forth between the other two, so be aware when pulling.
With these guys out of the way, you find yourself in the chamber with Blood Queen Lana'thel hovering over three dead vampire dudes (that don't sparkle.) As you approach she empowers an orb behind them that gives them life again. For this pull you can either charge in on Lana'thel and fight or pull back to the door and let them stand up before pulling...
THE BLOOD PRINCE COUNCIL!
For this encounter, you have 3 boss level NPCs to kill that all share one health pool, presumably because of Lana'thel's little orb trick. Only one of them is empowered at a time and that is the only one you can do damage to to affect the total health pool. The others that aren't empowered show at 1 or no health to indicate that they aren't the right target for you to be hitting at the moment. The standard way to do this fight is with 2 tanks in 10 man, possibly 3 in 25. If you just have 2, the first tank picks up Valanar and Taldaram (Lightning and Fire) and the other tank handles Keleseth (shadow). There are a number of other objects in the room to be aware of while fighting these guys. There are Kinetic orbs that drop from the ceiling. If they hit the ground, they explode doing damage and a knock back to players in their area. The way to keep them from dropping to the floor is by causing them damage, which sends them back up to the ceiling. They have a limited duration and can easily be managed by one pet class or two ranged as there are often 2 up at a time (possibly more in 25 man). If you're picked to do this job as a mage, I'd make sure Ice Lance is on your bar and ready. As an instant cast direct damage, it's perfect for this job. Move around the room so it's in your range and use that spell to knock the orb back up to the ceiling and safely away from the floor before resuming dps on the bosses. Keep in mind that DOTs do not work to keep the orbs up and neither do wands. Using long cast spells to do this job is unwise as you may find yourself far from the orb with little time to get it back up. Arcane Barrage is also acceptable, but it has a longer cooldown than Ice Lance does. If you happen to be in frost and the only ranged in a 10 man raid, you CAN do this by yourself. Set your water elemental to attack one of the orbs while you handle the other. You may not do any damage to the bosses, but your raid will be happy not to be asploded.
The second kind of orb in the environment you want to be aware of are the Dark Nuclei that spawn for the Keleseth tank to pick up. These things have very little health and follow threat rules so it's important that you don't hit them with any spells, AOE included. The shadow tank needs to keep 4 of them on himself when Keleseth starts doing his big bad shadowbolt when he's empowered by the orb. That's pretty much all you need to know about Keleseth. Be careful not to pull aggro when he's active, and don't steal the tanks orbs. If you happen to get one on you, let the tank know so he can pull it off.
The last little items to be aware of in the environment are the Shock Vortexes that Valanar summons while he's not empowered. They look like white tornadoes rising up from the floor and are targetted on a player, but once they become active, they affect everyone around them, knocking them back and causing damage. Don't stand next to one if you see one. Get far far away. Now on to the other two Princes and their mechanics.
Prince Valanar is always the first of the council active when you start the fight and thus always your first target. When he's empowered, he does an ability called Empowered Shock Vortex that does an area of effect around EVERYONE in the raid causing damage to the player and anyone in their vicinity and knocking back anyone close to them. The trick here is to be 12 yards from anyone else when he casts it, then you won't get hit or hit anyone. Generally, you spread out once the fight starts so that no one's standing on top of each other and the only people that have to worry about this are the melee and the shadow tank who's running around grabbing orbs. However, you may find yourself needing to move for another member of your group; keep in mind that healers and tanks should be given priority spots during this fight -- don't expect them to move for you. Use Blink if you have to to get out of the way.
Should the orb switch it's buff to Taldaram, you'll find yourself in a different situation. Taldaram summons a Empowered Flame Orb that follows a player as they kite it away from the raid. This orb has a certain number of stacks on it that need to be pulled off by other players being near it to prevent a big boom on the targeted player once it reaches him or her. Most raids have their melee follow the orb and the ranged and healers move into it's path to pull off some of the stacks. Should you be the targeted person, run away on a path that takes you through a lot of other players, but gives you enough distance to get a good ways away before taking the hit. It's also important to note that Taldaram has an ability called Glittering Sparks (sparkly vampire!) that acts as a cone effect in front of him, doing a DOT to the players in his path and also slowing their movement speed. You shouldn't be hit with this unless you're directly behind the tank keeping aggro on him. Don't be there.
The orb switches around between the 3 princes until their dead. You'll find they don't have a whole lot of health, but there's so much to pay attention to in the encounter that it's a nice present from Blizzard. Once their down, loot up!
Viable 10 Man Loot: Cerise Coiled Ring, Wand of the Ruby Claret, Pale Corpse Boots, Bloodsoul Raiment
Viable 25 Man Loot: Shadow Silk Spindle, Valanar's Other Signet Ring, Sanguine Silk Robes, San'layn Ritualist Gloves
After divvying up your spoils, head to either of the stairs on the side of the room and up and around to the top floor of this wing where you'll be face to face with
BLOOD QUEEN LANA'THEL!
Pew pew pew pew... pew... Ok, back to business. If you've done the Quel'Delar quest line then you're familiar with this former Blood Elf turned bad. She's working for the big bad guy now, and biting people along the way. She conveniently hanging out in a circular room with a big circle drawn on the floor for this raid encounter's setup. The tank will usually take her on the stairs where she's standing facing away from the raid. Note that the stairs behind you will turn into fire and doom if you remain on them once she's aggroed. Don't stand there. Instead, go into the circular room and pick a spot on the clock where no one else is standing so you're a little bit spread out, but can still hit the boss from your position. The big gimick in this fight is to DPS Lana'thel down before the enrage timer. Why is this hard, you ask? Because she has a lot of health and there's a lot of other minor distractions to keep you busy. The way to do it is to arrange your bite order in such a way that your best dps are bit longest. Players who are bitten by BQL and other raid members are given the buff Essence of the Blood Queen which gives you increased dps and heals you when you land attacks or spells. After 75 seconds of pure DPS bliss, you need to bite another person, otherwise Lana'thel will mind control you and use you to kill your own raiding buddies. Once it comes time to bite your bar will wipe, save for a single ability to bite another player. That player gets the buff and you're restored to your massive dps, allowing you to continue to blow her the heck up. Keep in mind that every 75 seconds you have to bite someone new. Hopefully your raid leader will have discussed a system of bites or a bite order with you previous to the start of the encounter, unless he or she is just quick enough to call them out as they come. It's important that the main tank is NEVER bitten as the buff also reduces threat caused by the players who are bitten and you generally want your tank to generate threat. Healers are ok to bite if needed, but it's not nice to make them run around looking for other people to bite, so make them your last set if you have to bite them at all.
The other little tidbits to pay attention to involve debuffs Lana'thel drops on the raid members at random throughout the fight and an air phase. The first debuff is Pact of the Darkfallen. In 10 man, these red squiggly lines link 2 players together, causing them damage while they're far apart. It links 3 in 25. The way to get rid of it is for all the players with the link to be close together so it drops. Usually this is done in the center of the large circle so it's easy to know where to go and in close range of the healers. The other debuff to be concerned with is Swarming Shadows. Remember way back when you were raiding in TOC and you hit Jarraxus and he put the green fire on you and you had to run? This is like that, only purple. Run around the outside of the room to avoid leaving fire for other raiders to get hit by.
A little ways into the encounter, Lana'thel will leave the tank platform and run into the middle of the room, flying up into the air and using Incite Terror to fear all the players in the room. This fear can be broken like any other with tremor totem or trinkets or Will of the Forsaken. After a moment of watching you run around like crazed aunts, Lana'thel uses Bloodbolt Whirl which causes extra damage to you if you're within 6 yards of anyone else in the raid, so spread out.
Other abilities I haven't mentioned that Lana'thel uses are Shroud of Sorrow, a constant raid damage debuff, Delirious Slash and Blood Mirror, which the tanks have to worry about but you don't, and Twilight Bloodbolt, which Lana'thel tosses at players occasionally and is the reason you stay spread out for the whole fight. Once you've bitten a couple of your friends and done some crazy pewpew DPS, BQL dies and gives you pretty sparkly purple things!
Since you've read about both the bosses in this wing, I'll cover the Weekly Quest for it as well.
Weekly Quest Number Four: Rescue Me... Before She Eats Me for Breakfast!
This quest becomes available to you as soon as you hit up Upper Spire. An NPC will join the fray once the portal is active and run across the bridge, waiting for you before the trash. Blood Quickening is the name of the quest. You basically have 20 minutes from the moment you start your first blood wing trash to kill the Princes and begin a successful BQL kill. Minchar is hanging upside down by shackles above her little room on the 2nd floor and as long as you start her encounter before the 20 minute timer is up and she dies, you're good. This gives you a little room for wipes, although hopefully you won't have any. Once you beat her, you turn in the quest, get your frosty sack containing badges, money, and possibly a gem or an item, and some rep.