The prize for this week's most-talked-about Sunwell boss goes to... an imprisoned naaru. Formerly resident in Silvermoon City, in patch 2.4 M'uru was captured by Kael'thas and taken to the Sunwell Plateau, where he now holds the place of fifth boss in the instance. After defeating the Eredar Twins (which gives you a handy teleport to their room) and clearing a few trash packs, you'll find yourself at the doorway of M'uru's circular room.
Initially a fairly unimpressive looking boss -- discounting the 'wow, it's a naaru' factor -- as the encounter gets into full swing you begin to marvel at its clockwork complexity. Killing M'uru isn't the end of the fight, either; instead of dying, he transforms into a void god called Entropius, who you have to burn down as quickly as possible. It's a very intense encounter, even post-nerf.
Phase 1: M'uru
During Phase 1, four separate things are going on that require attention from different parts of the raid group. At the start of the fight you simply see M'uru himself in the centre of the room, but as the phase progresses, you have to deal with adds entering the room from either doorway and large void sentinels which create void spawns when they die. Additionally, M'uru spawns a large void zone in the centre of the room every so often which brings with it eight dark fiends that need to be dealt with.
It's a hectic fight, but the general approach is to divide your raid into individual groups to deal with the various mobs in the encounter, and thus simplify the fight by having people stick to their defined role throughout. When you reduce "deal with two different waves of adds, sentinels, spawns and M'uru" down to "kill these three mobs", it becomes a lot easier.
Generally raid setups for M'uru involve six healers -- one on each tank and two on the raid, since there's constant raid damage from Negative Energy, helping out on tanks during dangerous moments. Shamans excel on this fight since their bloodlust is a huge DPS buff for phase 2, but each healing class has its pros and cons here -- bring your best performers as long as they're not all the same class. You'll also want three or four tanks, and assorted DPS, which we'll talk about in a minute.
Blood Elf Adds
These spawn three at a time every 60 seconds, running in from either entrance to the room (the doorway that you first enter from, and the blocked-off hallway opposite). The general method is to have a tank at either doorway picking up these mobs. You can use a warrior, paladin or feral druid -- play to your tanks' strengths. Since you're tight on healers for the encounter, it often helps to have a mage on either side polymorphing one of the adds.
The waves consist of two Shadowsword Berserkers and one Shadowsword Fury Mage; the mage can be interrupted and has a self-buff (instant cast spells doing 50% more damage, but cannot move) that can be spellstolen. The berserkers deal dual-wielding physical damage and have a self-buff called Flurry (increased attack and movement speed) which can be temporarily negated with disarm, although feral tanks in particular find that stacking a high amount of avoidance means that it's hardly noticeable. The adds are also stunnable.
In order to control the adds, you need to kill each wave before the next wave spawns. This requires assigning 3-4 dps to kill each side; as there's a fairly tight DPS requirement here, you need to go for synergistic groups, usually melee or hunters (as your casters will be busy elsewhere). This depends on your usual raid composition, who your strongest players are, how good your tanks are etc -- having DPS pull aggro often leads to a wipe, so it might take a pull or two before your DPS and tanks are comfortable with each other.
An alternative strategy used by Drow removes melee from the encounter completely and simply tanks and AoEs the blood elves. It's an interesting approach, but not one that'll work for many guilds due to the fact that melee occasionally like to have raid spots. However, its novelty is worth a mention.
These giant voidwalkers hit fairly hard and can spawn from any of six locations in the room. They have a shadow-based AoE aura which means you don't want them near your raid members, but instead tank them off to one side -- it also means you need to be fairly fast picking them up, so they don't run off after healers and AoE your raid to death. Misdirection rotations can really make the difference here, or failing that, a lot of fast communication.
Void sentinels cast a spell called Void Blast which does a lot of damage and slows the tank's attacks; fortunately, it's spell reflectable, so many strategies involve using a warrior tank on void sentinels to remove void blast from the equation.
When the void sentinel dies, eight void spawns appear which have much weaker melee attacks but cast an AoE shadow bolt volley. The easiest way to deal with these is to use a protection paladin to AoE tank them. There are a couple of different approaches when it comes to sentinels and spawns DPS, though; generally you will want at least three warlocks assigned to these, with their primary job being to DPS Void Sentinels and then cast Seed of Corruption on spawns.
Some strategies choose to only AoE spawns every other wave, leaving the paladin tanking multiple sets at once, which gives the warlocks more time to DPS M'uru; you'll have to figure out what will work best for your guild, it'll depend on how many warlocks you have, how easy the paladin is to heal, how much mobility you need and how much DPS time you're getting on M'uru.
Another way to deal with void sentinels and spawns is used by SK-Gaming (thanks, Neg!) and simplifies this part of the fight. Instead of having a separate tanks for void sentinels, a protection paladin tanks both sentinels and spawns with the aid of a shaman's grounding totem to absorb void blasts. This relies on misdirections being used to ensure that every sentinel gets to the paladin, as there's less mobility than with the two tank approach; however, it frees up a raid spot on a DPS-reliant encounter. Each approach has its merits, so again, you will have to figure out what works best for your raid.
M'uru and Darkness
The naaru himself is in a fixed position and untankable, and generally every DPSer burns him down as much as possible at the start of the fight while tanks get ready to grab their adds. However, once mobs start spawning, there isn't much free time left to DPS M'uru himself -- your ranged DPS (especially shadow priests) will do the majority of damage to him while your melee focus on the side adds.
Darkness is cast every 45 seconds and fills the central ring in the room with a void zone that deals damage and makes you unhealable, but it's easily avoided; also on Darkness spawn, eight Dark Fiends spawn inside the circle which can only be killed by dispels or purges. This is easy enough if you have a shadow priest DPSing M'uru, as they can Mass Dispel the fiends. Depending on the time until the next Darkness you can do tricks such as AoEing the void spawns and side adds near M'uru to ensure extra damage is dealt to him, but with the recent health changes this isn't necessary (beyong speeding up the encounter a little).
As you can see there are a lot of things that can go wrong in Phase 1 and it will take many wipes until everyone's perfected their jobs. Initially you might run into 'beginner' errors such as not dispelling Dark Fiends or having nobody ready to pick up a Void Sentinel, but as everyone learns the spawn points and timings it becomes a lot smoother. However, dying due to aggro on side adds or uncontrolled mobs is always a concern, so you can't get complacent even if the boss is technically on farm.
Phase 2: Entropius
When M'uru dies he becomes Entropius, a large voidwalker-style 'void god'. This part of the fight is a DPS race, so you want to ensure you have the best possible transition between phases -- i.e. don't kill M'uru when you have two new waves of adds, a void sentinel and void spawns still alive! Ideally you'll have no adds left at the transition, although you can deal with a spare blood elf or two.
During this phase there are a couple of things to look out for: negative energy, singularities, dark fiends and void zones. Negative energy was present in the form of random chain-lightning style raid damage in phase 1, but in this phase it becomes more deadly, increasing in frequency and damage as the fight goes on. As well as dealing maximum damage, raid members need to be aware of their own health and stay alive as long as possible.
Void zones spawn on players throughout the phase and must be avoided as they deal damage; they also summon a single dark fiend, like the ones in phase 1, which needs to be dispelled or purged as soon as possible. On top of these, Singularities are spawned which deal some damage but more importantly bounce players around the room -- your tank needs to move Entropius so that melee do not get disturbed by singularities, while ranged members need to be aware of their position and move when necessary. Getting disrupted by a Singularity cuts out valuable DPS or healing time and could make the difference in this fight.
In summary, phase 2 involves avoiding void zones and singularities and using your maximum DPS consumables and cooldowns to burn Entropius down while attempting to stay alive. Phase 1 is a lot more repetitive and generally accepted to be harder, so after you regularly get into phase 2, a kill shouldn't be too far off.