This is a fight with a lot of movement and changing targets, which isn’t ideal for rogues. Still, we serve an important role in this fight, and we get interrupt duty again! The fight is fairly complicated, so you’ll have a lot to think about.
This is a two-phase fight. In phase 1 the boss doesn’t attack any single target and has no aggro table, so you can attack her whenever you get a chance. She throws around random shadowbolts, which you can’t avoid, and death & decay which you don’t stand in. Your attention will be on the adds. (on 25-man she also mind controls random raid members, who must be CCed)
When the fight begins, adds will spawn from one side of the room. In 10-man there will be three adds from one side of the room. In 25-man there are seven (spawns from both sides and one from the back). All tanks and dps will work on taking these out quickly, because new ones spawn every 60 seconds. Your role is to maximize your burst damage here and take out adds quickly.
There are two kinds of adds – Adherents and Fanatics. They can go through three “phases” – when they first come out they are Cult Fanatics/Adherents. Lady Deathwhisper can randomly buff them, making them Empowered Fanatics/Adherents, and they can kill themselves (doing AoE to all around them) and get reanimated to be Reanimated Fanatics/Adherents. The main annoyance about this is that it makes it harder to write targeting macros.
Fanatics are melee and Adherents are casters. The key features we need to know about them are that Fanatics cleave, and Adherents can put up a spell reflect shield. In addition, in their Reanimated versions, Fanatics are immune to physical damage, and Adherents are immune to spell damage. Put these factors together, and it points to rogues focusing on Adherents.
When my guild did this, we had melee focus on Adherents and caster dps focus on Fanatics. That worked well. If your raid is heavily melee or caster, then you’ll need to adjust your strategy or your raid composition. Burn down the Adherents as quickly as possible. You can thrown some interrupts in there to help reduce the damage on the tanks if your healers are having trouble. If any add gets Empowered, focus on that one as a priority. If you finish quickly you can help with the Fanatics, or run over to Lady Deathwhisper and get to work on her.
When the adds are down and the next wave has not spawned yet, get to the boss and attack. When you’re attacking the boss, she takes no damage! That’s because she has a mana shield. All damage reduces her mana rather than health. You have to burn through that, and it will take a while. Keep on the boss until the next wave of adds spawn and go take out Adherents again. Return to the boss when done, and repeat until her mana is gone and the mana shield drops. Remember to drop Rupture on the boss when you leave her so it ticks while you are on the adds.
When the mana shield falls, there is a threat reset, so use Tricks on the tank to help with aggro. From this point, it is just a tank and spank burn phase. She has a long-cast-time Frostbolt that does a fair chunk of damage, so Kick that. You should be able to interrupt all of them. There will still be Death & Decay (and Mind Control on 25-man) as before, so watch for those.
Use Tricks of the Trade liberally but carefully. The tank will get a stacking debuff called Touch of Insignificance which reduces their threat. If you’re not paying attention you can easily pull aggro here. Tricks will help you give the tank more threat, but if you are tank-swapping them make sure your Tricks goes to the correct tank.
Vengeful Shades will spawn around the raid during this phase, and they blow up if they reach anyone. Keep an eye out for them and run away. Remember, a dead rogue does zero dps.
If you can get through the adds to phase two you’re basically home free. That is, IF you can beat the 10-minute enrage timer.
In the 25-man raid you’re hoping for the Cultist’s Bloodsoaked Spaulders. Also, there is Heartpierce, a slow MH dagger with a unique proc. Does the interesting proc make up for the lack of sockets and stats? Time to hit the spreadsheet.