This mount can only drop while in 25-man Heroic Icecrown Citadel against the Lich King. It is advised that the smallest number of players engage the boss to raise the chances of the individual obtaining the mount.
During phase 1 the Lich King gains an ability called Shadow Trap. Dealing with it is similar to dealing with Defile - everyone needs to move away from where the targeted player was standing, as the Shadow Trap will cause everybody standing in it to be knocked off the platform after about 3 seconds trigger time.
A simple way to keep track of this spell is to have a tank hold the boss in the center of the ring (where Highlord Tirion Fordring stands prior to the fight starting) with any melee dps, while all ranged members stack on one of the prongs seen on the floor. If another tank is available, they should tank the Shambling Horror and Drudge Ghoul as they are summoned off to the side of the circle. If the Shadow Trap goes out on the melee group, they should shift around the center of the ring. If the Shadow Trap is placed on someone in the ranged group, the entire group should move to the next prong, going in a consistent direction. If it's on the tank with adds, they should move away from the spot but stay in range of the rest of the group, since Necrotic Plague will still be going out.
Necrotic Plague is a disease that inflicts damage every 5 seconds for 15 seconds. If the effect ends (it expires, host dies, or is removed with a spell), Necrotic Plague will jump to another target within 10 yards. If no players or adds are within 10 yards, Necrotic Plague will disappear. However, it should be noted that whenever Necrotic Plague jumps, The Lich King gains a stack of Plague Siphon, which will increase the strain on healers.
If the effect is removed with a spell, Necrotic Plague will lose a stack; if the target dies while affected, it will gain a stack. For this reason, players should run into the open area away from the rest of the group to be dispelled, or to run to the adds being tanked away from the rest of the raid group.
Players should also be aware of the ability Infest, which is active until phase 3 of the fight. This is unavoidable and must be healed through, but to avoid taking additional damage, players should bring the boss under 90% health as quickly as possible.
Transition Phase 1
When The Lich King hits 70% health, he will run to the center of the platform and start casting Remorseless Winter, an AoE attack that extends in a wide circle around the boss. At this point, all players should run to the outer ring of the platform, as it is the only safe space. It's important for players to group in the same area on this outer ring to contend with the mechanics during the transition phase.
One or two ranged players should be designated to kill the Ice Sphere that appear, which drop near the boss before moving toward a randomly chosen target - if everyone is in the same area, this will be the same general direction and make it easier for the ranged players to deal with. When not killing the Ice Sphere, the designated ranged players should attack the Lich King to shorten the duration of phase 2.
The remainder of the raid group should focus their attention on killing any remaining Shambling Horror and as well as the Raging Spirit that are summoned.
When the transition phase is ending, the Lich King will stop casting Remorseless Winter and begin to cast Quake. All players should immediately move back into the main platform (off the outer ring) - once he finishes this cast, the outer ring will drop off, taking anyone still on it with it.
During phase 2, the boss will periodically summon Val'kyr Shadowguard, winged minions who grab a random player and carry them to the edge of the platform before dropping them off the edge. It's imperative that all dps players immediately stop attacking the Lich King in favor of slowing and damaging the Val'kyr Shadowguard. Although they must be killed in Normal difficulty, in Heroic when their health drops below 50%, they will drop their targets and fly above the raid. The Val'kyr Shadowguard will drain health from the raid, healing themselves using Life Siphon, but will not pick up another player. The key to surviving this mechanic is not only to get the Val'kyr Shadowguard down to 50% quickly, thereby keeping all of the raid group members on the platform, but also to have high dps on the boss to limit the number of Val'kyr Shadowguard that leech health from the raid.
The boss will also cast Defile during this phase. It is a purple puddle that he will place on the ground under a random player that grows as players stand in it, and can expand to cover the entire platform. To combat this, players should spread out prior to Defile being cast and immediately start running away from the spot where the targeted player was standing - it can help to already be moving when it is cast to lessen the amount of time one stands in the puddle, but be careful not to run into it if behind the player.
Transition Phase 2
When The Lich King hits 40% health, he will enter the second transition phase, running back to the center of the platform and start casting Remorseless Winter. At this point, all players should run back towards the outer ring of the platform, which will reappear. It's once again important for players to group in the same area on this outer ring to contend with the mechanics during the transition phase. The spells cast and adds summoned are the same in this transition phase, so players should be accustomed to their assignments.
Remember to call out reassignments if any of the tanks or designated ranged players have been killed.
When the transition phase is ending, the Lich King will stop casting Remorseless Winter and begin to cast Quake. All players should immediately move back into the main platform (off the outer ring) - once he finishes this cast, the outer ring will drop off as before, taking anyone still on it with it.
During phase 3, the The Lich King will use Harvest Souls on everyone in the raid group. Instead of one player, the entire raid will be sent inside Frostmourne. There, instead of Terenas Menethil fighting the Spirit Warden, they will see a lot of Wicked Spirit that float above the floor. If a Wicked Spirit reaches a player, it cast Spirit Burst - similar to the Vile Spirit found in Normal difficulty. There are also "bombs" that fall slowly from the ceiling and explode when they hit the ground, the damage from which should be healed through. When all the Wicked Spirit are dead, Terenas Menethil will cast Restore Soul on all members of the raid, regardless of when or how they died, and send them back to fight the Lich King. Upon exiting, players will find a swarm of Shado-Pan Enforcer flying over the platform - killing them is of lower priority than dealing damage to the boss, but they will cast Spirit Burst if still alive after 30 seconds, which deals some damage to players in range.
Watch out for Defile when you get out, as you will all be stacked around the boss and it will come out soon after exiting Frostmourne.