Mindbender (or Solace and Insanity)
In a nutshell, this boss is a DPS check (depending on ilvl) with a bouncing ball mini-game. The enrage timer is 6 minutes and you get 4 rounds of the bouncing ball mini-game. In terms of difficulty, I want to say this fight is hard, but at the same time it's relatively easy if you are geared and also managed to get a lot of bounces in the first round of the mini-game. The fight is definitely easy for T15 BIS geared. In terms of mechanics, there pretty much just 1 to deal with and that's the bouncing ball mini-game I mentioned earlier.
This is a stacking buff the boss gains periodically throughout the fight. Each stack increases his physical damage by 10% and attack speed by 10%.
Void of the Unmaking:
The core mechanic of this fight. The boss will summon a ball once every minute which starts to move towards the outer edges of the room away from the boss. The player should prevent this from happening by stacking on the ball to diffuse (or bounce) it. Upon diffusing, the ball will deal some moderate damage that increases with each diffusion (So don't bouncing till the point of no return). Once the player has bounced the ball, it will start to move in the opposite direction as well as triggering a small cooldown that prevents it from being bounced for a short period. The player must then follow the ball to get a second bounce in and the cycle repeats until the player no longer wishes to bounce (due to excessive damage from diffusion or too many Focused Anger stacks from the boss). Eventually the player must let the ball diffuse into the boss. This will reset his Focused Anger stacks as well as applying an increased damage taken debuff on the boss. The amount of increased damage taken is 5% for each bounce the player triggered before letting the ball hit the boss. The boss will also go into a short AOE phase before the entire cycle repeats.
Dealing with Void of the Unmaking:
Due to the high DPS requirements (assuming you are not T15 BIS), it is essential to bounce as many times as possible in the first round rather than in later rounds. This is because at the start of the fight he will summon the ball before he starts stacking his Focused Anger, whereas in later rounds he will spawn the ball after Focused Anger has already reached a few stacks. Also keep in mind that although more bounces means a bigger damage taken debuff, but at the same time, more bounces means more DPS downtime. This is especially important in the later rounds. For example at the start when the boss has no stacks of the debuff, losing 20 seconds DPS time to bounce the ball is ok. But later on when the boss has 10 stacks (50% increased damage taken), losing the same 20 seconds just to bounce the ball to increase to 13 stacks (65% increased damage taken) may actually have a negative impact. You may actually need to consider if it is worth losing 20 seconds worth of doing 150% dps in order to be able to do 165% DPS for the remaining 1 minute left on the fight. Plus, there is also the factor of increased healing required due to more bounces, which causes even higher DPS downtime. Of course this example is only there to provide some insight and the numbers may not actually true.
With that said, I personally went with 7 (tried going for 8 but the ball accidentally hit the boss) bounces on the first round, 3 bounces on the next two rounds, and just 1 bounce on the final round. This may not necessarily be the most ideal number of bounces for each round, and other priests may come up with better options in the future.
Defensive cooldowns are definitely needed in the first round if you are aiming for high number of bounces. I used up all my available defensive cooldowns aiming for 8 bounces in the first round (even though I got 7 by accident in the end), so I'd say getting up to 9 will be extremely hard (but probably doable with glyphed VE), and 10 is outright impossible. Other than that, defensive cooldowns can and should be used whenever healing cannot keep up. Also Flash Heals should be used conservatively.