How To: A Tankadin Guide to Yogg-Saron

The Yogg-Saron fight has three phases, and the tankadin tasks for each are a bit different.

Phase 1:
In this phase, you need to kill guardians, which spawn from moving clouds, close to Sara until she dies (she takes about 12% of her health in damage per killed guardian).
We usually have three tanks (or two tanks plus a dps warrior or death knight in defensive stance/frost presence). The raid is positioned close to the entrance. The two main tanks pick up guardians and tank them close to the raid until their health is low. Then the guardians are kited close to Sara where a small group of ranged dpsers finish them off. As soon as a guardian is dead, the tank runs back to the raid to pick up a new guardian. The third (off) tank is picking up lose guardians and holds them away from the healers until one of the main tanks picks them up.
Things to look out for: Stepping in the clouds will make an extra guardian spawn, and if too many of them are up at the same time, the raid risks to be overrun by them.

Phase 2:
Tentacle time! Yogg-Saron appears in the center of the room and three kinds of tentacles spawn. Portals will open, letting raid members enter Yoggie's sick, perverted mind. The phase will end when Yogg-Saron's brain has taken enough damage from the people inside his mind.
However, a tankadin is more useful outside, so you won't be going into the portals.
As mentioned, there are three types of tentacles spawning outside.

• The Crushers channel a nasty debuff called Diminish Power, which reduces the damage the raid does with 20%. It stacks up to 4 times if you have 4 Crushers up (which is a REALLY bad idea). The channeling can be interrupted by hitting it with a melee attack. Sounds easy enough, right? Well, the Crusher also gets a stacking buff called Focused Anger everytime it is hit that increases damage done by 3%. This buff can stack up to 99 times. This makes it impossible to tank the tentacle, since it will one-shot anything with 40 or so stacks of the buff. The tankadin thus needs to "joust", i.e. run in, hit it, and quickly run out. Try to avoid getting more than 30 stacks of Focused Anger, so that the tanks don't risk dying.
• The Constrictors Squeeze raid members. When being squeezed, one takes 7500 damage every second. Only way to get people out of Squeeze is to kill the Constrictor. As a tankadin, you can bubble your way out of this. You can also use Hand of Protection on someone else who is squeezed. Save your HoP for a melee that needs to enter a portal, or for someone who otherwise would die.
  
• The Corruptors spread four kinds of debuffs: a magic one, a curse, a disease, and a poison. Help the healers to cleanse these, especially the mana-draining poison.
  

The tankadin's duties in phase 2 are to joust the Crushers to interrupt Diminsh Power, cleanse the debuffs from the Corruptors, and help people get out of the Constrictors' Squeeze (either by killing the tentacle or using Hand of Protection).
Things to look out for: Your Sanity might take some hits due to Brain Link, Malady of the Mind, and Psychosis. Make sure to refresh your Sanity at one of the Sanity Wells, because if your Sanity hits zero, you are mind controlled for the rest of the fight (and your raid will mock you for it when they kill you).

Phase 3:
In the third phase the ranged memebers of the raid will dps Yogg-Saron. The melee dps will spend their time killing off the Immortal Guardians that spawn every 10 seconds. These guardians hit very hard at full health (they will one-shot everything except the tanks), but hit like a smurf when low on health. The guardians should be tanked at least 20 yards away from Yogg-Saron since he casts Empowering Shadows on random guardians, and letting Yoggie heal himself is a bad tactic.
Yogg-Saron also casts Lunatic Gaze every 12 seconds. All raid members must face away from Yogg-Saron when this is being cast.
We bunch all the raid up away from Yogg-Saron, preferrably close to a Sanity Well. All healers face away from Yogg-Saron during the entire fight. We use two tanks: one to pick up the spawning adds before they kill someone, and one to tank them until they are at 1% health (when Thorim will kill them off). Melee dps single targets the guardians to burn them down as fast as possible, since they hit so hard at full health.
As a tankadin, you can either be the tank picking up spawning adds with your many ranged pick-up tools (Exorcism, Avenger's Shield, Hand of Reckoning, Righteous Defense, Judgement), or you could AoE tank the adds while spamming Righteous Defense on your fellow tank, to keep all adds on you until they die.
Things to look out for: Again, keep an eye on your Sanity.

It is an intense and long fight where simple mistakes will make the encounter very difficult.

Good luck!