So, first of - Phase one: You've got this round room, with this unattackable 'avatar' standing in the middle casting all sorts of nasty stuff at the group (debuffs increasing the damage you take, dotting you with something that'll kill you twice over if not healed, etc).
The only way to defeat the avatar is to kill about 10 or so tentacle beasts that spawn at increasing rates. Not so bad you say? Well, there are gas clouds moving erratically around the room; if anyone touches one, it'll immediately spawn another tentacle beast. This wouldn't be so difficult, if not for the fact that they will stand and cast things that reduce the amount of healing the group takes by 25%, on top of the fact when they do die, they inflict massive damage to anyone nearby. It's a test of coordination (dragging the half dead mobs to the avatar so they can die there, avoiding clouds, interrupting etc).
So, phase one is fun, but now you have phase two: During this phase, the actual boss appears. The only thing visible above ground is the tip of his head, and a fair amount of tentacles. You still can't damage the boss directly, so first off you have the large tentacles: These guys continually channel an ability that reduces the damage everyone does by 20% and it stacks. The only way to stop the cast is to go in and melee the tentacle. The problem with this? Every single attack done to a large tentacle enrages it, to the point where they will do about 300% more damage, and attack 300% faster. They have no ranged attack (thank goodness), but in order to stop the debuff, you'll still need someone to run in and melee... Here's hoping they don't get whacked...
So then the little tentacles; these guys are fun. There are a wicked amount of them spread throughout the room, and they're all busy casting away. They'll slow your attack and movement speeds, they'll put a dot on you that can and will kill you if not dispelled within 12 seconds, they can poison you with enough potency to leave you stunned every 2 seconds. On top of this they also do lingering damage that will drain your mana. (Very necessary for healers to function).
Well, we're still not done with tentacles. Now you've got constrictor tentacles, which do exactly as the name states: They appear out of the blue and wrap around someone inflicting insane damage every second until that person is released. Not so bad if the whole group is nearby, but that rarely happens.
So, tentacles are fun, but you remember that 'avatar' back from phase one? Yeah, she's back, and unattackable. But she's going to have her way with you anyway. So, she'll randomly do unavoidable damage to players, spawn this giant laser beam that MUST be avoided by everyone (for the sake of your health), and then the really gritty stuff: She'll randomly fear a group member (make them run off uncontrollably). Not so bad in itself, but when fear wears off, if there was another group member nearby, the fear jumps to them. On top of that, you've got Brain Link. A link appears between two players in the room, they have to run to each other as fast as possible, as the link won't disappear until they do, and they're taking damage as long as they're up linked. This is especially fun if someone is linked to another caught in a constrictor tentacle across the room.
Okay, so lots of damage going out. Priority initially is to kill tentacles (they respawn), but then the actual boss (tip of the head thing) will spawn 10 portals around his head (in a 25 man raid), which have to be entered as soon as possible. This also has to be coordinated before the encounter starts, so everyone is ready to get to their portal without wasting any time.
So you thought getting out of that hell whole of a room was nice? Think again. Inside the 'Brain Room' as it were, you have to kill more tentacles. These guys look meek, but for every bit of damage you do to them, they do 60% of it back to you, so you have to pace yourself so the healer can keep up. Not to mention, as you're running around this room trying to kill the tentacles, there are also these ghostly skulls (unattackable) hanging around. If your character looks at one - literally faces a skull - they take damage, AND lose sanity (more on that later).
So, you've got the tentacles inside down, good, but you only have 60 seconds total inside the brain room, and the tentacles aren't even your priority. Once they are all dead, a door opens up revealing Yogg-Saron's brain. This is the time where everyone goes all out doing as much damage as possible to it. When you've got about 15-5 seconds left on the timer, you have to take a portal back to the main room. If you don't? Well, you're teleported back, but you've been mindcontrolled, turned against the group, and made to do 100% more damage, alongside a buff of 300% health. These people MUST be killed, and they cannot be revived later during the fight, as they will just return to being mind controlled.
This whole phase repeats itself internally until the brain is down to 30% health. If you don't get it that low within about two Brain Room encounters (I think it was 2, maybe 3?) then you're in some serious trouble by that point.
So now you've moved onto phase three: Yogg exposes more of his head in one direction and the party needs to move to stand in front of him. During this phase, any leftover tentacles from phase 2 still exist, which can really, really hurt. As well, you've now got the mobile tentacle beasts from phase one spawning and doing their own thing: The more health they have, the more damage they do, so having multiple of them spawning is painful for the tanks and healers. They also drain life (which can be interrupted) which restores a decent amount. On top of this, Yogg is also empowering any of them which are within 20 meters of him - regenerating their health at a quick pace to the point where if more than one or two are hit with this at a time, it can be devastating for the group.
The fun thing here also, is when you're doing heroic mode Yogg-Saron, you're not getting outside help like you do in regular mode. So in normal mode, a beneficial avatar is present that would kill any tentacle beast that's at 1% health. If you're doing heroic, the beasts do no die. So someone has to tank all the beasts and keep them far enough away from the boss so they do not recieve buffs. It's easy to get upwards of 10 of them on the tank. (If I recall correctly).
So when the tentacle beasts are under control, (they don't stop spawning however) the raid focuses on attacking Yogg-Saron himself. He doesn't target anyone specifically, but every 12 seconds, he'll cast lunatic gaze. This will only affect someone that is starring at Yogg when he casts it, it reduces sanity dramatically and does damage.
-----------------------------------------------
Now, about sanity: at the start of the encounter everyone in the raid obtains an unremovable debuff called Sanity. Throughout the entire fight, this sanity meter (which starts at 100) is going down. Nearly any enemy attack that does damage reduces sanity. Looking at Yogg, the avatar's attacks, tentacles, etc. If you get to 0 sanity, you become mind controlled. This is the same as if you don't exit the Brain Room quickly enough. The only way to get sanity back is if you stand in the light beam one of the beneficial avatars puts out in the room. Now... on heroic, she's not around. So every single point of sanity counts.
-----------------------------------------------
So you've got avoiding gas clouds and blowing up avatars down, you can kill tentacles and bash brains till you're red in the face, you can handle the debuffs, sanity loss and insane amount of tentacle beasts in phase three.... Now what do you face?
An enrage timer of 15 minutes.
Yeah.. I hate Yogg.
(But hey, getting that boss down was probably the most satisfying thing I ever experienced in WoW, and I would gladly go back just for fights like that.)
