Relevant Talents used:

Body and Soul
From Darkness, Comes Light
Twist of Fate
Halo

iLvl: 491

Another boss down after just 1 blind attempt. After finishing this fight I believe I can cut the encounter duration by at least 30% if I get some more practice.

This boss itself is healing intensive (for obvious reasons) since your objective is to heal the boss to full. That's 6 million healing (10man normal), not including the extra slight damage the boss takes as well as the damage you will take throughout the fight. Additionally adds will spawn frequently throughout the encounter which must be defeated.

The encounter starts with the boss being on 50% (6M health) and is friendly to players. Healing the boss to 100% (12M health) will complete the encounter. The boss will periodically spawn portals. These will port the players into a different phase where they can collect orbs that increase mana regen as well as healing power. However, going into these portals while soloing will instantly reset the encounter. Therefore the only purpose these portals serve is to prevent death after a wipe is called.

There are several types of adds that will spawn throughout the encounter. These are all trivial to kill for a level 90 priest. However the suppressor adds that channel energy on the boss must be killed first as they reduce the healing intake on the boss. All the other types of adds have their own mechanics, but for a level 90 character, there really is no need to pay attention to most of them. There is one mechanic however that you need to watch out for. The caster adds can sometimes spawn a water geyser at your feet which you must move away from to avoid taking massive fall damage.

Being a fight that requires killing a lot of adds as well as massive heals on the boss means that mana is the main problem. Shadowfiend and Dispersion alone won't be adequate for mana regen. Therefore Vampiric Touch must be up for as much as possible on as many adds as possible. Ideally, Vampiric Touch should be the primary source of damage for the encounter (excluding Mind Sear/Halo for the little worms). The more ticks VT lands on an add before it dies, the more mana returned. On the other hand, VT should not be the only spell you put up on every add as this will not kill the adds fast enough before new ones spawn. So it's best to seek an ideal balance between VT and other spell usage when killing the adds.

In terms of talents, Twist of Fate really shines on this encounter as this is just as much of an add fight as it is a healing fight. Wouldn't be surprised if uptime is close to 90%. Keep in mind that ToF won't proc if you one-shot an add that had full health.