The Obsidian Sanctum

This ten-person raid is of moderate difficulty, with most of the success hinging on executing the strategy rather than being overgeared. It is located underneath Wyrmrest Temple, inside the Chamber of Aspects itself.
It is infamous for having one of Blizzard's first boss-difficulty sliders; Sartharion herself is surrounded by three Twilight Drakes. Killing all three will give you a pleasantly tactical Sartharion encounter; leaving one alive gives you a challenge; leaving two alive gives you hell; leaving all three up is utter chaos and the hardest encounter pre-Ulduar.

Trash Notes
You will notice immediately upon entering OS that Sartharion is directly in front of you. Patrols of large dragonkin range around the room, and the Twilight Drakes are at the left, right and rear of the cavern. Your raid will do a full circle around the outer edge, clockwise, until you return to the entrance portal having killed everything.

Q: If we leave some trash alive and engage Sartharion, they don't aggro as well do they?
A: Oh yes, they do.

So kill them all. The groups of four red ones have nasty fire spells, and usually should be split between the OT and MT evenly to ensure a managable damage income. The pairs of green-grey ones do a hideous curse; bring a mage or druid (not optional). After a few kills you will encounter Vesperon.

VESPERON
The important thing for these drakes is to ensure each raid member has been given clear instructions. Just diving in is a recipe for tasty, nutritious annihilation. During the fight Vesperon will open a portal to the "Twilight Realm". While there players will take DoT and be unable to see or help the people in the real world. Inside the Realm will be a mob that must die before the fight can continue.
The MT and a healer stay outside, while the entire of the rest of the raid goes in and downs the mob. Ensure nobody stands in the void zones, and that the healer who is staying out knows he/she is staying out.

Continue killing your way around until you reach Tenebron.

TENEBRON
This guy is slightly different. He creates a portal just the same, and the MT and healer must stay out. However inside the portal are whelp eggs. The DPS have only a short amount of time to kill them all, or they will hatch whelps into the real world, and make life difficult for the tanks.

Some more trash, then Shadron.

SHADRON
Another Twilight Drake, another portal. This time the mob in the Twilight Realm shields Shadron, making him invulnerable. Kill the add, kill Shadron, finish the rest of the trash and form up in front of Sartharion.

SARTHARION, THE ONYX GUARDIAN Video
Standard dragon mechanics; cleave and tail swipe. She should be tanked facing sideways, with the MT almost in the lava. The rest of the raid should be either stabbing her in the side or standing 10-20 yards off. Her moves are not all that: a fire breath that only the MT should be exposed to, and some AoE from the volcanoes that is ignorable.

The key to this fight is the Flame Walls. At regular intervals throughout the fight, there is a raidwide emote: "The Lava Surrounding Sartharion Churns!" A wall of fire will rise out of the lava on either the left or right side of the platform (randomly), and sweep across to the other side. Deadly Boss Mods will warn you when it is incoming. The entire raid needs to move into one of the gaps in the walls, bearing in mind that the gaps are in different locations in the left and right Flame Walls. Anybody who is hit by a wall as it sweeps across will take damage and spawn an add. These adds are not too troublesome individually but in large numbers they will cause a wipe. The OT should pick up these adds, and DPS should kill them. If another Flame Wall comes and adds are themselves hit by a wall, they become much more powerful (+400% damage); thus the OT should place the adds in the gaps as well. While the adds are a consequence of flame wall impacts, they also spawn randomly throughout the fight as well. OT be ready at all times to take them off healers.

As the fight progresses people will become accustomed to shifting back and forth into the gaps. The MT especially must take care not to accidentally allow Sartharion's tail to swipe and stun raid members as a wall sweeps across.

When Sarth reaches 10% HP she enrages, large numbers of adds will spawn, and everything goes a bit nuts. Blow her to pieces quickly, and that's that.