The Obsidian Sanctum

Sartharion's lair can be found in the depths of Wyrmrest Temple in the Dragonblight, "guarded" by these clowns:

Abilities

Sartharion:

Flame Breath (Fire) - Doh, don't stand in front of him.
Tail Lash (Physical) - Doh, don't stand behind him.
Cleave (Physical) - Doh, don't stand in front of him. Again.
Will of Sartharion (Buff) - Increases Sartharion and the Twilight Drake's health by 25%.
Twilight Revenge (Buff) - Whenever you kill a Twilight Drake, Sartharions Physical damage and attack speed increases by 25%.
Pyrobuffet (Fire) - Inflicts Fire damage to an enemy and increases Fire damage taken for until cancelled. I guess this will be used when noone is in melee range or at the end of a wipe.

Tenebron:

Shadow Breath (Shadow) - Doh, don't stand in front of him.
Shadow Fissure (Shadow) - Spawns blue bad stuff, you get hurt after 5 seconds.
Power of Tenebron (Debuff) - Tenebron's presence increases Shadow damage taken by all enemies by 100%.
Hatch Eggs - Every ~45 seconds Tenebron will spawn Eggs in the Twilight Realm.

The eggs will hatch 20 seconds later, spawning 6 Sartharion Twilight Whelps:

Fade Armor (Debuff) - Reduces your armor by 1500 for 15 seconds, stacks up to 10 times.

Shadron:

Shadow Breath (Shadow) - Doh, don't stand in front of him.
Shadow Fissure (Shadow) - Spawns blue bad stuff, you get hurt after 5 seconds.
Power of Shadron (Debuff) - Shadron's presence increases Fire damage taken by all enemies by 100%.

Every 30~60 seconds Shadron will summon a disciple in the Twilight Realm:

Gift of Twilight (Buff) - Increases Fire Damage of Sartharion and makes him immune to damage.

Vesperon:

Shadow Breath (Shadow) - Doh, don't stand in front of him.
Shadow Fissure (Shadow) - Spawns blue bad stuff, you get hurt after 5 seconds.
Power of Vesperon (Debuff) - Vesperon's presence decreases the maximum health of all enemies by 25%.

Every 30~60 seconds Vesperon will summon a disciple in the Twilight Realm:

Twilight Torment (Debuff) - You take Shadow Damage whenever you attack outside of the Twilight Realm as long as the debuff is up. Also Shadow Damage taken is increased by 75%.

You can enter the Twilight Realm through portals:

Twilight Shift (Debuff) - Reduces the target's damage by 25%. Inflicts Shadow damage every 1 second.
Twilight Residue (Buff) - When you leave the Twilight Realm you are immune to Shadow Damage for 3 seconds.

There's also environmental damage in the area:

Magma (Fire) - While you swim in the magma you take Fire Damage every 2 seconds.
Flame Tsunami (Fire) - Large flame walls, you're slowed and taking Fire Damage when hit.

And you'll have little Fire Elementals spawning:

Molten Fury (Buff) - If the Fire Elemental gets hit by the lava waves, it's health, damage and size increases.

And finally, you have Fire Cyclones in the outer magma area:

Lava Strike (Fire) - They throw bombs into the area that deals Fire damage in a small area.
Burning Winds (Fire) - Powerful winds knock players back and can literally tear elementals apart.

The following tactic has been written by Lancar:

Class requirements

3-4 Tanks, 1 shaman for heroism.

The pull

The tank with the highest HP is usually designated as the Tank for Sartharion himself. He will have Sartharion misdirect-pulled towards him and position him right where the raid gathered at the start, facing the island sideways. The raid runs to the furthest part of the island while the pull in progress. The pull is executed like this in all cases which you want add participants in the encounter. If not, its easier to just tank him where he stands.

Positions and strategy

The raid gets most of the island to move around on freely, apart from the front and back of Sartharion (for obvious reasons).
This encounter can be done using anywhere from 0 to 3 extra drake adds (Tenenbron, Shadron, Vesperon), increasing the difficulty but also the potential loot rewards. The strategy is divided into 4 parts, depending on how many adds you want. Kill off the ones you dont want to participate prior to pulling Sartharion.

Sartharion Alone:

Just Sartharion himself. This is the recommended starting point for any guild or group who has no prior experience with this boss. The encounter is fairly straightforward, with only a few twists.
Tank Sarth where he stands. At regular intervals there will be an warning message saying that the lava around Sartharion churs, thats the cue to watch for the incoming lava waves on the sides. These waves may appear from either side, but not both at once. They also have spaces between them that you must run for to avoid getting hit. If you DO get hit, you will take alot of fire damage, get slowed and take loads more fire damage over time. If too many raiders gets hit, it is likely the healers will not be able to save everyone, or run dry on mana doing it.

The other thing to watch out for is the blue circles appearing on the floor (Shadow Fissure) every now and again. These have a shorter fuse than the ones at Kel'thusad, so if you dont immediately drop what you're doing and run out from them ASAP you will die.
Lastly, there will be small fire elementals spawning regulary. Another tank will have to pick these up, and the AoE people will take care of them before they get too many to handle.

Sartharion & One Drake:

You will have to pick which add to leave alive before engaging. The easiest one would be Shadron, since his ability matters the least.

Do the pull described at the top of this document. After a while, Shadron will fly down. A Tank picks him up and tanks him at the side of the island.

The raid will have to avoid his breath attacks while DPSing him down. After he is dead, most everyone goes into the portal that has spawned to enter the Twilight Realm. There will a dragonkin inside that will need to be killed before tackling Sartharion since he projects a damage shield on him that prevents all damage. After the Twilight Realm is clear of mobs the raiders inside it will be returned to the normal plane, and they can proceed with Sartharion as per the "Sartharion + none" tactic described above.

Shadrons aura will cause all fire damage taken by the raid to double, so don't get hit by lava waves. You might not survive it.
His deciple will, apart from making Sartharion immune to damage (and threat generation from the MT) will also cause him to do 50% more damage with his breath attack. You'll need a sturdier MT than in sarth + none.

Sartharion & Two Drakes:

Now it gets interesting.
The easiest drakes to leave alive for this is Shadron and Vesperon, but you might want Tenenbron part of the mix instead of Shadron in order to get some practice fighting him if you havent before. This tactic will assume you're taking Shadron, though.

The encounter begins exactly as for Sarth + 1, but a while after Shadron has landed Vesperon will land as well. This guy acts in pretty much the same way Shadron does, but with one notable exception: his dragonkin in the Twilight Realm needs to be killed right when it spawns.

Proceed to DPS Shadron when he lands. When Vesperon lands, watch for the raid warning saying that he has created a portal. Enter it (it should be near his landing spot), and kill anyone inside. When you get out of the shadow realm, return to DPS Shadron. Repeat this until Shadron is dead, then switch to Vesperon.

Don't forget to take care of the fire elementals when they get too many, avoid lava waves and blue circles.
In addition to double fire damage taken from Shadrons aura, Vesperons aura will cause everyone to have 25% less maxHP. There is much less room for mistakes.

Sartharion & Three Drakes:

Not defeated yet. Watch this space...

Normal Difficulty

The fight doesn't change on Normal Difficulty.

Boss Kill:

Sartharion & One Drake:

Sartharion & Two Drakes: