From Darkness, Comes Light
The final boss of Icecrown Citadel has 3 phases, as well as a transitional phase in between each of them. The most challenging phase is in fact phase 1, which served as the major brickwall that prevented me from soloing it back in level 85. At level 90 however, the fight is no longer all that challenging, even phase 1. Though phase 1 still remains to be more challenging than the other 2 phases.
During phase 1, the boss will summon adds as well as applying the Necrotic Plague debuff. The debuff is actually required to kill the adds but unfortunately it is never applied on the tanking player. So to get the debuff, shadowfiend is used as a temporary tank so that the debuff can be applied. The debuff deals around 50k damage every 5 seconds for 15 seconds and after it expires, it will jump to the nearest add. The boss also summons a Shambling horror with about 2 million health and hits just as hard as the boss. The best way to kill Shambling Horrors is with Necrotic Plague. The boss also places another dot called "Infest" which deals increasing damage and only expires when health is above 90%. In terms of DPS, it is preferable to get the boss down to 70% before the second Shambling Horror spawns. Vampiric Embrace and Desperate Prayer are used to counter Infest.
The first transitional phase will start when the boss reaches 70%. It is preferable to have most of the adds dead by this time. It's okay to have 1 Shambling Horror alive, but have 2 alive can be a bit problematic. During this phase the boss will summon ice spheres which fixates and moves towards the player, touching these will knock the player back, typically off the platform, causing a wipe. Fortunately they have under 10k health and can easily be 1 shotted. An even easier way to deal with these orbs is to stand at the throne, so touching the spheres only causes knockback against the wall. The raging spirit adds summoned during this phase is the top kill priority. They have an occasional 5 second silence ability which isn't too detrimental unless there is more than 1 Shambling Horror still alive. The transitional phase ends after 3 Raging Spirits have been summoned.
Phase 2 is a lot easier than phase 1. The only mechanic to watch out for is the infamous Defile, which is a pool that grows in size and power the more damage it deals to players. Adds and Infest no longer applies in phase 2. When the boss reaches 40% the second transitional phase begins. This is the same as the first transitional phase, with the exception that 1 extra raging spirit will spawn with no extra time to kill it.
Phase 3 is still faceroll. Vile Spirits are not that threatening for a level 90. Additionally, they only travel a certain distance before they despawn, which means they can also be outranged. Keep in mind that Defile still applies in phase 3. Get the boss down to 10% to win the fight. He will kill you with Fury of the Frostmourne ability, but that is intended. In the video I survived it with Dispersion.