(The illustration is unfinished and a bit messy still but you should be able to get the general idea of positions and the counter to eye-beam and dark glare from it.)

Note: The abilities are compiled from various sources such as wowwiki, videos and some german sites. Tactics are just a rough draft of what seems plausible when looking at the abilities and videos. It is my impression that the tactics actually come down to everyone playing smart and understanding the encounter. Stay alive and help others to stay alive.

C'Thun is a 2-phase fight. Phase I faces the Eye of C'Thun. In Phase II the battle is against C'Thun itself.

The Eye and C'Thun itself cannot be tanked. It doesnt have an aggro list and uses only ranged abilities. It is also immobile.

The fight takes place in two locations. Outside and inside C'Thun's stomach.

IMPORTANT: Do not run past the door to C'Thun's room, do not even take a peek. It will aggro you and instantly kill everyone.

Abilities

PHASE I

Eye Beam:
Green Beam originatin in the pupil of C'thun's Eye. Random target (unlimited range), hits for 2000-2500 nature damage and jumps to nearby players within 10 yds, doubling it's damage with each jump. Can be resisted. (Certain death for the 3rd or later player affected if it jumps)

Dark Glare:
Red Beam (or rather "wall") originating in the pupil of C'Thun's Eye. Random target (unlimited range), hits for 50k shadow damage anyone who passes through the beam. It lasts 35 seconds and moves slowly 180° either clock- or counterclockwise.

Eye Tentacle:
Every 45 seconds 8 Eye Tentacles spawn in circular fashion around C'Thun at the edge of the black smoke. Each of them has 2300 hp and their only ability is a 750 shadow dmg per tick Mind Flay on a single random target. They are vulnerable to stun, counterspell, shock, silence, shieldbash, etc.

Claw Tentacle:
At random intervals Claw Tentacles appear in random locations throughout the room. Upon spawn, they deal 1500 physical(?) damage and knock back all players within 10 yds of their spawn-point.

PHASE II

Damage Absorbation:
By default C'Thun absorbs all damage.

Eye Tentacle:
(Same as Phase I but on shorter timer) Every 30 seconds 8 Eye Tentacles spawn in circular fashion at the edge of the black smoke around C'Thun. Each of them has 2300 hp and heir only ability is a 750 shadow dmg per tick Mind Flay on a single random target. They are vulnerable to stun, counterspell, shock, silence, shieldbash, etc.

Giant Eye Tentacle:
Every 40 seconds a Giant Eye Tentacle spawns in a random location in the room. It has roughly 40k hp and deals some minor melee damage. In addition it posess the same "Eye Beam" ability C'Thun's Eye in Phase I had --> Random target, hits for 2000-2500 nature damage and jumps to nearby players within 10 yds, doubling its damage with each jump. Can be resisted. Vulnerable to stun, silence, etc.

Giant Claw Tentacle:
Every 40 seconds C'Thun randomly picks a player from the raid and spawns a Giant Claw Tentacle at his feet. This causes a knockback on nearby players (10 yds) and deals 2500 nature damage on each of them. The Tentacle has 65k hp and deals significant melee damage. Tankable.

Every 10 seconds a random player will be swallowed by one of the Giant Tentacles and thus become teleported into C'Thun's Stomach.

(Stomach) Digestive Acid:
The teleport to C'Thun's stomach drops the player into a green liquid that causes a debuff "Digestive Acid", which deals 150 nature damage every 3 seconds. It stacks every 5 seconds independent of where once stands inside the stomach. Cannot be dispelled.

(Stomach) Flesh Tentacle:
Inside C'Thun's stomach, two of these tentacles can be found. They both have 25000 hp and deal moderate melee damage. If both of them get killed, C'Thuns body on the surface is vulnerable to attacks for 45 seconds. After that period the Tentacle respawns.

(Stomach) Teleport Back:
Inside C'Thun's stomach, three islands can be found. Two for fighting the Flesh Tentacles, and one with a black surface. Stepping onto that surface teleports the player back outside and he can resume the fight.

Process

PHASE I

0) Setup 8 self sufficient groups that always stay together (in range)
1) Spread out, stay at least 10 yds from each other (get the C'Thun Warner mod (/ctw)). Evenly all around C'Thun.
2) Kill the mini-tentacles asap (they can kill rapidly). Eyes first, Claws second.
3) On a dark glare (red beam), run to your alternate position on the opposite side of the room, don't run through the beam.

PULL: Make sure everyone is mounted and paying attention. Upon signal everyone runs into the room together. As soon as the first player enters, everyone gets into combat. Now everyone got 10 seconds to reach his designated position before C'Thun starts casting Eye-Beams.

Once inside odd groups branch out left and even groups right. While running everyone has to make sure not to overtake those in front of him, or to get close in the first place. Some people might dismount early, so be aware.

10 Sec after entry, C'Thun starts it's eye-beams. By that time everyone should be in position.

GROUP SETUP + POSITIONS: Each group has at least 1 melee, 1 healer and 2 ranged/casters plus one random class. One melee player in the group is made the group-leader and everyone in the group follows his movement. he dictates to moment when the group runs in on the pull and functions as main assist for the eyes. He is also by default the only melee to attack the groups designated Eye Tentacle. The group itself spreads out circular while making sure that no two players stand in conjunction with each other towards C'Thun. This way a Dark Glare can never hit two players simultaneously and make dodging it easier. --> See illustration.

GENERAL: 45 seconds after the pull Eye Tentacles will spawn for the first time accompanied by a Dark Glare. Killing these Tentacles quick (within 3-4 seconds of their spawn) while avoiding the Dark Glare is the critical ability a raid has to learn. The Dark Glare rotates 180° either clock- or counterclockwise killing everyone who passes through it. Once the Glare stops another wave of Eye Tentacles spawns accompanied by Eye Beams from C'Thun. Dark Glare and Eye Beam phases will alternate with Eye Tentacle spawns in between.

DARK GLARE starts on a random player and turns slowly either clock- or counterclockwise. Before the beam starts, the eye turns red and stares into one direction for about 3 seconds with some black smoke in the pupil. This is the cue for everyone in that direction to clear the area immediately and get a safe distance from the beam. Note: We don't use mod-announcement for this as we have seen its much simpler and more reliable to have players just watch the eye turn red and react to that.

The beam is slower than a running player, so no matter which way it actually turns, one is safe as long as he keeps running. The Dark Glare turns 180° and then stops.

During a dark glare, small eye tentacles will have to be killed quickly or their mindflay will make it impossible for some to run away from the beam.

To maintain the spread out positions, upon each dark glare groups switch to the opposite side of the room (which mean effectively rotation the whole raid 180° with the beam).

Afer a Dark Glare another wave of Eye Tentacles Spawns and another Eye Beam phase starts.

EYE BEAM: In order to conter eye beams everyone has to stay spread out 10 yds from each other. The Eye Beams have a tendence to hit those players who are not spread properly first, so there is no margin for error really. If all groups know their two alternate positions accross the room and they internally keep the positions up as illustrated above this is no big deal.

It gets tricky when some players fell behind during the preceeding Dark Glare and couldn't get in position in time. Its best for them to stay where they are in that case and find a safe spot nearby until the next dark glare. Running over others is bad, very bad.

EYE TENTACLES usually spawn at the beginning of Dark Glare and Eye Beam phases. During Eye Beam they are killed as people move past them. DoTs, instant spells and melee hits work well here. If the Glare is still at a safe distance, stopping briefly is the way to go. During Eye Beam the Tentacles are killed by the melee primarily as they are quicker to output 2500 dmg than casters, who nevertheless help. Its possible to target eye tentacles before they are fully spawned. Note: Targetting Tentacles at a distance (> 20 yds)is not possible. In the positions illustrated above, each group is responsible for one Eye spawn. Note: We dont use an announce mod for these as its proved to be more reliable to have people just pay attention to whats going on around them.

A macro of great utility is: /target Eye Tentacle it has a "built-in" priority that suits the priority of the encounter. It targets the nearest Eye Tentacle --> Giant Eye Tentacle --> (Giant) Claw Tentacle.

PHASE II

0) Same setup as in phase I
1) Spread out as in phase I
2) Warriors grab a Giant Claw tentacle when it spawns
3) Kill the Eye Tentacles asap --> then Giant Eyes --> then Giant Caws
3.5) Dont buch up, stay spread, only deal with whats in your half of the room.
4) When swallowed, jump on land, do as much damage to the RIGHT Flesh Tentacle as you can without dying. Then Teleport back up. Healers help the DPS inside to stay a bit longer. More than one healer isn't needed inside.
5) Once both Flesh Tentacles in the Stomach are dead, C'Thun is becomes vulnerable to attacks for 45 seconds. In that event all remaining Eyes on the outside get killed quickly. Then DPS goes all out on C'Thun.
6) After the weakened ends. There will be no Eye Tentacle Spawns within 20 seconds. Within those 20 seconds all remaining Claws should be killed rapidly.
7) Repeat this process until C'Thun is dead. If your DPS is good it should not take more than 3 weakened phases to kill it.

Ramblings

In Phase II its important that Mages all around the room remain in position to Counterspell any Giant Eye Tentacles asap.

Split the room in North/East/South/West. Announce where the Tentacles spawn by using these simple directions. Everyone in that "hemisphere" then goeas about killing the spawn.

Have someone "Talk" the raid through the encounter. Remind people whats coming next (Glare, Small Eyes) and what to do. In Phase 2 remind to spread and annoucne where a tentacle spawns, which one to kill, which to stun, when to switch. Also constantly updating the Stomach status and generally making everyone aware whats going on so they can use their best judgement.

Melee stay at maximum melee range to get more distance between your next neighbours.

A macro of great utility is: /target Eye Tentacle it has a "built-in" priority that suits the priority of the encounter. It targets the nearest Eye Tentacle --> Giant Eye Tentacle --> (Giant) Claw Tentacle.

Priests should be positioned near the middle of the group, behind the first row, to reach as many as possible.

Healing in this fight is unlike any other bossfight, and I want to point out a few things which most probably know by now:

1. You heal your own group, but must be ready to crossheal the groups that are on your left and right side. If you use emergency monitor, you can chose that it only shows ppl in these 3 groups, or you can just organize the raidlayout on your ui for easy targeting of your 3 groups (recommended). Phase 1 is not manaintensive.

2. Eye Beams deal about 3k damage, and quite often it hits the same target twice in a row with only a second interval. This means death for squishy classes (mages and ourselves). Only a quickly casted shield can save them (us) in these cases. And we really shouldnt die from eye beams.

3. Eye tentackles. It takes a few seconds before these start to mindflay after they spawn. The mindflay requires heavy healing, included shields etc, and we are better off using a less amount of mana on offensive spells against the eyes.
Main priority is to kill these asap, and we do that by first casting Mindblast and then smite (w00t nuke!). Do not waste mana on SW:P, because thats effectively 470 mana down the drain, and if we do things right it wont ever tick even once. We shouldnt die of mindflay either.

4. Claw tentackles
Do not waste mana on killing these. Wand them.