Sartharion

The Obsidian Sanctum has 3 mini-bosses plus Sarthurian - a Northrend version of Onyxia - he even drops [Dragon Hide Bag]! No attunement or resistance gear required though, which is a plus.

Composition:
2 MT, 3 healers, 5 DPS.

Mini-bosses
Sarturian has three twilight drake lieutenants. The easiest way to do the encounter is to clear them all beforehand, any that you leave alive before engaging Sartharian will join the fight and add unique abilities that make the fight harder, but also drop an extra loot item for each one left alive. Completing Sartharian + 3 drakes on 10-man is currently the most difficult raid encounter in the game.

When engaged before Sartharian each of the drakes opens a portal with a 'Disciple' add in it, and whilst the add is alive gains a special ability.

Vesperon
Has approx. 975k health. While the disciple in the portal is up everyone gains a debuff that lowers max health by 25%. He does a frontal cone breath attack for 7-8k fire damage, and a tail swipe that does 3-4k damage and a 2 sec stun. He also spawns shadow fissures on the ground that do 6-9k damage after 5 seconds. You do know the first rule of raiding, right?

Rather than deal with the disciple through the portal we brute force Vesperon. Typical dragon-style raid poistioning: ranged and healers stay well back, tank positions the boss sideways on, melee stay around back legs.

Tenebron:
Same attacks. When the disciple in the portal is up he gains an aura that increases shadow damage by 100%. In the portal he hatches eggs which spawn whelps, which will come out of the portal and aggro onto the raid. MANY WHELPS!!!! We will send 1 tank (pref pally) and 1 healer to OT the whelps and all the DPS will stay on Shadron until he's down. The whelps have a stacking Fade Armor debuff that drops 1200 armor from the tank, so he'll need more healing than normal.

Shadron:
Same breath and shadow fissure attacks. When the disciple in the portal is up he gains and aura that reduces damage taken by a lot. MT + 1 healer stays outside, OT + 2 healers + all DPS enters the portal and zergs the Disciple of Shadron. Whilst in the portal the raid takes shadow DoT, hence the possible need for a second healer, we'll see. When the disciple is down everyone goes back to the boss. Shadron will continue summoning portals and disciples until he's killed.

Sartharian
Now that his lieutenants are down we go lay the smackdown on their head honcho.

Sartharian is a typical dragon, has 2,500,000 hp, he has a frontal cleave, a frontal cone breath attack for 9-11k fire damage, and a tail swipe for 3-4k + a 2sec stun. During the fight there are 'tidal waves' of fire that sweep back and forth, with a gap in-between that the raid needs to spot and position themselves to pass untouched. Lave waves alternate left-to-right and right-to-left.

During the fight small groups of 2-4 adds will be spawned, they need to be picked up and focussed down. If the adds are hit by a lava wave, they will be buffed and do 400% damage, so it's nice for the OT to keep them out of the lava waves.

At 10% he does a minor enrage, summoning more adds, we just OT these and concentrate on finishing the boss.