Meat Shield versus Carpet Bombing
The third phase of the Lich King encounter is one of the most difficult parts of the encounter. As your raid continues to be targeted by Defiles, neccessitating your raid to continually reposition, the Lich King will call a number of Vile Spirits into being above him. They hover for a few seconds before swooping in towards the raid, exploding and harming as many of your allies as they can.
There are currently two accepted strategies for dealing with them, which I will called Meat Shield and Carpet Bomb.
The strategy I see advised often involves massive amounts of AoE on the Vile Spirits to make sure most die before they come anywhere near to the raid. This also involves kiting the ones that aggro on you due to your DPS, and this is the situation where Mind Sear will shine. Using the Glyph of Mind Sear is advised when using this strategy, to ensure that you catch as many of them as possible. As for the kiting bit, well that usually involves dotting them up and running away.
This is also the tactic where you will see good use of your Speed-increasing enchant on boots. Besides its usefulness in dodging Defile, it will allow you more leeway in outrunning the Spirits.
The downside of this tactic is that doing this depends entirely on the DPS being able to deal enough damage to enough spirits to mitigate the damage on the raid. This means that it has a relatively high casualty rate among the DPS. On the upside, individual DPS will skyrocket – it allows for obscene meter-padding. Not my favorite, but it is nice to let people hang loose and see the numbers fly. This helps motivate people as well.
Another tactic is that the entire raid moves as the Vile Spirits are summoned, and one or two dedicated soakers run into the mass of Spirits each time and keep themselves alive as they get hit by 10K blasts from each Spirit. This is an ideal job for Paladins (shield), Shadowpriests (Dispersion) and Mages (Ice Block) where you become immune or nearly immune and can take down the Spirits with impunity.
This is the tactic we used when we managed to down the Lich King, and it seems to work well. As a Shadowpriest, you want the Glyph of Dispersion when using this tactic. Not just for being able to soak more often (raid utility, yay!) but also because you want to use it to survive the Lich King attempting to devour your soul.
Downside of this tactic is that you spend some time not dealing damage, and personal DPS will suffer. However, with a minimum of coordination survival becomes quite easy in this phase, leading to a quicker kill. However, it also allows more DPS on the Lich King, since only one or two people need to go to the Spirits each summoning. This is a bit of a balancing act, but when you take turns it will end up good for all involved.
In this phase, the choice of tactic will depend on your DPS. With high AoE dps and few people who have major soaking cooldowns available, Carpet Bombing is advised. But when you are still learning the fight and have a number of people in the raid with soaking cooldowns, this tactic will help in quickly learning the fight and making the kill.
I think that the Carpet Bombing tactic will be the way to go once you are sporting hard-mode gear, where DPS rises to such monstrous proportions the Spirits won’t even reach the ground. But when struggling in this face, having healthy and observant soakers will definitely be a supreme tactic.