The encounter itself is very flexible and dozens of guilds have come up with different strategies, so we're not going to dictate the "must-follow" hard and fast rules here. Instead, let's take a look at the bosses and their abilities and provide you with the tools to come up with your own plan of attack, along with some tips and tricks.
Before we get stuck in to the Twins themselves, let's touch on the trash beforehand. Yes, trash! After two bosses with minimal-to-no trash you'd expect a trash clear to be a welcome sight, but from the first time someone accidentally pulls the first pack, you'll realise that the majority of the trash is a lootless gauntlet. There are two ways to deal with the gauntlet: the Shadowsword Commander at the end controls the mobs' spawning and the fiery imps that continually bombard you, so some guilds just ride through the gauntlet, resurrect and reform, then kill the Commander. Having not tried this we can't vouch for it, but it seems like a quick (but expensive) way to despawn the gauntlet.
The other way is simply to kill everything, which is easy enough with practice. Make sure someone is up front exploding the imps so they don't bomb the raid; many mobs are sheepable, or you can take a paladin tank and spam-heal while it tanks everything. Once you get to the Commander, the gauntlet is over, and you get a few pulls that actually do drop loot before you get to the Twins themselves.
Any attack from Sacrolash applies a shadow debuff, Dark Touched, which reduces healing effects by 5% and stacks up to 20. At 20 stacks, healing done to you will be reduced by 100%, so you must clear this debuff before it gets that high. Any fire damage taken clears the debuff (note, Hellfire and similar class abilities have been hotfixed not to work here -- only environmental damage).
Her other abilities:
- Shadow Blades: a shadow attack hitting 3 random targets within 20 yards for around 3k damage. Applies Dark Strike, a slowing effect.
- Shadow Nova: a targeted nova with a 10 yard radius, dealing about 3.5k shadow damage.
- Confounding Blow: confounds the tank, dealing up to 10k shadow damage and removing them temporarily from her aggro table. Lasts 6 seconds.
Abilities: Grand Warlock Alythess
The debuff Alythess applies is Flame Touched, a randomly targeted fire DoT that starts out at 250 damage every 3 seconds and stacks up to 20 (5k damage every 3 seconds). Any shadow damage taken clears the debuff (again, class abilities and items don't work for this).
Her other abilities:
- Pyrogenics: a self-buff that increases fire damage done by 35%. Can (indeed, should) be spellstolen or purged.
- Conflagration: this targeted spell has a 3.5 second cast time; once Conflagration lands, the target is confounded for 6 seconds and takes 1600 fire damage every second for 10 seconds. They also deal fire damage to those nearby. Can be removed with the PvP trinket once it has landed.
- Blaze: hits the highest on aggro (i.e. the tank) for up to 7.5k fire damage, leaving patches of fire on the floor that deal damage when stepped in.
- Flame Sear: a fire attack hitting up to 5 random targets with a fire DoT, dealing 650 fire damage every second for 6 seconds.
If the Flame Touched and Dark Touched debuffs aren't managed correctly, the raid will soon wipe to large stacks of Flame Touched ticking down on some people while others are being hit by shadow damage but immune to healing from a large stack of Dark Touched. Fortunately, the ranges of the twins' attacks and the logic behind them mean that you can position the raid and the twins to take care of a lot of this for you -- and ensure that raid members can manage their own debuffs to a large extent.
People killing the twins at range will likely be out of range of the shadow blades, and your positioning might mean you don't get hit by shadowfury or the Shadow Images at all, but you can ensure that someone in the ranged camp gets Shadow Nova and stand close enough to get hit by it. Alternatively, you can get closer to Sacrolash and hope for a Shadow Blades or Dark Strike. Those taking a lot of shadow damage, such as the Sacrolash tanks, can jump in Alythess' Blaze after a Confounding Blow to clear debuffs.
Conflagration and Shadow Nova have been observed to follow threat tables to some extent. Conflagration often hits the person third on Sacrolash's aggro table, and Shadow Nova the person second on Alythess', though the base observations are that the abilities hit anyone from the second to sixth on the aggro table. It's therefore quite important to control aggro and be careful with spells such as Earth Shield and Prayer of Mending that generate global threat -- if your Alythess tank gets this sort of healing threat right at the start of the fight, it's quite possible that you'll see a Conflag hit it. Similarly, you want Shadow Nova to hit your ranged camp, so watch Alythess' threat table.
Being able to control these two abilities and where they land mean it's possible to control debuffs, too. However, raid damage from Flame Sear, Flame Touched and the Shadow Images can still be a killer -- there's a lot of damage hitting in a very short space of time. Trinkets such as the Battlemaster's, granting a temporary health boost, survival potions, stamina gear, healthstones, class abilities and even resistance gear are all useful here to some extent.
Due to the huge amount of raid damage, most kills have taken up to 11 healers, although people have managed with 10. Raid healing on this fight is a tough job and you really want healers that can step up to the mark and perform, although tank healing isn't exactly an easy ride either. AoE healing abilities are nice, but there is also a lot of single target damage such as Conflagrate or Sear that needs to be healed up very quickly.
You'll need at least two tanks for Sacrolash. Due to Conflagration picking a target from the top of the aggro table, it's possible to have one tank Conflagrated while the other is Confounded, leaving Sacrolash free to hit the third on threat. Tanks should wear their PvP trinkets and watch Confounding timers, but a third tank makes this a lot smoother. Any combination of warrior, druid or paladin tanks will work, but bear in mind that the bosses crush. It's also possible (due to lag, presumably) to get hit after Confounding Blow lands -- this is burst damage of up to 20k, so your tanks need to be well-geared.
The Alythess tank can be either a warlock or warrior; most strategies use a warlock but it's been successfully done with a warrior tank too. Due to Blaze, a warrior tank makes DPSing her a little more interesting for melee, whereas a warlock can stay at range.
The rest of your raid is obviously made up of DPS; it isn't a DPS race, but you do want players who perform, and who are on the ball as well as able to keep themselves alive.
It's possible to kill the twins in either order. Killing Alythess first gives Sacrolash the ability to Conflagrate; killing Sacrolash gives Alythess Shadow Nova. These are the only ways to reset your debuffs once one twin is dead. Killing Alythess first was the most common strategy until a guild called Fusion published a video of the 'reverse' strat, with Sacrolash first; this tactic has been widely adopted and conveniently gives more loot.
Depending on the order in which you kill the twins, you can position in any number of ways to take advantage of line of sight and range. The positioning given in Fusion's video for Sacrolash first, for example, ensures that the shadow images have the maximum distance to run to reach the raid -- minimising damage from them and almost entirely eliminating Shadowfury. (Sacrolash moves around fairly often so positioning with her can never be exact).
Alternatively, if you choose to kill Alythess first, one approach is to tank Sacrolash upstairs with simply tanks and healers nearby to maximise the distance between her and the raid. You'll probably want the warlock to come in range of Shadow Blades, and the Sacrolash tank healers will be in range of a lot of shadow damage so they might need shadow resistance gear.
These are just a couple of examples -- by considering the debuffs and your raid setup you can work out the best positioning for yourselves and choose the strategy you want to use. It will take some time to get your raid well practiced at staying alive, and you will wipe repeatedly to things like: people not moving (fast enough) out of the raid with Conflagration; people getting Flame Sear and dying to that and Flame Touched; people running off the ledge at the top and aggroing the twins; tanks getting gibbed by Confounding and melee damage.
Analysing why you wiped and getting people not to repeat mistakes is a fine art, but once you get there it's a case of having the random number generator on your side to some extent, managing debuffs by controlling threat and staying calm in the face of a lot of raid damage. Good luck!
Elitist Jerks (and benefactors' thread)
Fusion video (Sacrolash first)
Last Resort video (Alythess first)