THIS IS IT! The fight we waited all expansion for! The chance to defeat the Lich King is here!
I've commented previously that it is a wonderful fight and that Blizzard did a great job of designing the encounter. Most fights are either 1) dps tanknspanks, 2) survival, or 3) gimmicks. In Icecrown, the only other fight that combines several of these elements is Putricide. On Lich King, you'll have to maintain steady dps on the boss, while bursting adds down, surviving several different potential raid-killers, and coordinate target-switching and slowing abilities.
Lich King is the FINAL boss in Icecrown Citadel, the ultimate challenge remaining in normal and heroic modes (no secret boss here). YOU are NOT prepared!
Pull/Phase One (necrotic plague): DO NOT summon your army in this fight. There are too many adds that can be taunted by them. Go ahead with the rest of your dps cooldowns, including DRW/Gargoyle/ghoul. You'll be behind Lich King, but be sure you know where the Shambling Horrors are being tanked so you can run to them asap if you get necrotic plague. As unholy DK, I pestilence to keep Ebon Plaguebringer up so all the adds take more damage from plague etc, but otherwise we ONLY dps Lich King in P1. There's no need to dps adds, they'll die from necrotic plague. You'll want to max dps just to keep the fight shorter, but also time it so you don't get a third Horror.
P2 transition (Raging Spirits): Know when it's coming so you can immediately run to the edge. You don't want to burden healers while taking too much damage from Remorseless Winter. That, combined with an unfortunately timed infest and/or pain and suffering, can kill you if you're not careful. I usually take the opportunity while running to refresh bone shield and I anti-magic shield to ensure my health stays high. In addition, if you have a ghoul up, call him with you to the edge, or he'll eventually die. You can put him on passive to call him, then tell him to attack the Raging Spirit. If you merely put him on defensive, he'll run back to Lich King so be careful to keep him on Raging Spirits. You'll need the dps on the adds, and you want your ghoul to survive. Pump the dps on the Raging Spirits, you'll want at least 2/3 (on 25-man) down before the transition is over.
P2: Carefully run back in for Phase 2, keeping dps on any remaining spirits. Pestilence to get diseases back on Lich King. Pestilence again the moment Valkyr are targetable. Some DKs Death & Decay here, but I prefer to save my runes for pestilence, slowing, and dps on my designated Valkyr. Groups do this one of two ways; having 'teams' assigned to specific Valkyr, or having a priority - killing them in order. We found better success with the latter. In any case, maximize burst dps on your Valkyr to get it down quickly, but be prepared to move out of the way of Defile. A Defile dropped in the path of Valkyr is an almost guaranteed wipe. Also be prepared to use your chains of ice as needed to slow Valkyr. Death Grip does not work on them. I use my gargoyle again (or other cooldown) when the first Valkyr appear because you want to ensure they all die, along with remaining Raging Spirits.
P3 transition can be scary, but if you've learned P2 transition, you can do it! We pop Heroism/Bloodlust here to try to get the Raging Spirits down; probably gargoyle/drw/ghoul will be up again at this point. Also use your haste pot here. Since there are more Spirits, pay attention to the kill target and burst it down, switching crisply from target to target. You may well have two Spirits up when the transition is over, be careful to stay out of their way, and kill them when you can (but don't miss defiles, etc).
P3 (Vile Spirits): Run back in, but don't get greedy! You can still wipe with defiles or vile spirits (thankfully, no infest)! Your ranged can kill Viles, but as melee, just stay away! You shouldn't have to worry about the enrage at this point (at least on normal), so don't try to run to the tank too soon, and stay away from others so you don't blow each other up. In between, dps LK as you can. Something I missed originally: when you get Harvest Soul on you, if you are properly healed, you'll survive (yaay!), but be sucked into Frostmourne. Inside, (as a DK) you'll have to dps the add, and interrupt the long cast of Soul Rip - which will kill Terenas if you don't. You have 60 seconds to kill the add and escape. Once it's dead, you'll pop back out and rejoin the raid.
P4: as you know, get him to 10% and you win! This is a time for screaming and cheering while you watch the events and/or cutscene. DO NOT RELEASE! You'll be rezzed at the end and get to kill him for free from that point.
To sum up: DPS the boss only in P1. In other phases, dps the adds in the correct order as quickly as possible, then dps the Lich King. Stay away from the many things that will kill you. Spread diseases. My dps on this fight is poor compared to others, but all that matters is surviving and controlling adds!
Congratulations, you've just completed Icecrown Citadel!
These 'guides' to dps DKs have been on normal - as nuances crop up and I find ways to deal with them, I'll try to post on heroic modes as well...
Good luck to all in Icecrown!