Just got through reading everybody's comments above, and wish someone had summarized it all. So here's my shot at it. Did I miss anything?

The Obsidian Power Source drops from the Obsidian Sentinel inside Uldaman.

The back entrance to Uldaman is at 67,44 in the Badlands in some island of rocks next to some ravine with elite dragons. Or if you don't use Cosmos, like me, then it's about 2/3 of the way across the map and a bit north of the equator. (Or if you're OCD, it's 67/100ths of the way across from the left edge of the map and 44/100ths of the way down from the top edge.)

1. He reflects all "targeted" spells.

This means:
* Use ONLY melee, wand and AoE spells (and, duh, Frost Nova and Mana Shields)
* If you use fire attacks, make sure to use a fire ward (dunno if you need one otherwise...)

This also means you only need to bring along one friend if you choose the right class:
* A shaman or priest can remove any of those reflected magical effects if needed (and, duh, heal you...)
* A tank can melee without worrying about all that reflection crud
* If you pick someone who is at least level 45 (preferrably higher), then you probably won't wipe at all.

2. He spawns "mini-me" adds, but gets weaker each time he does it.

This means you'll have an easier time of it if you:
* Kill each new (lower level) add before returning to the boss.
* Kill the boss after he has weakened himself by spawning a handful of adds. (Didn't see a specific number, but it sounds like you should wait til he spawns at least 3 or 4...)

3. He's sitting at the back instance entrance, behind the closed doors.

This means:
* You can run out of the instance screaming like a little girl, heal up and run back in again as often as you like. p

I'm about to do this quest. Being the wimpy little girl that I am, I intend to take advantage of ALL this info!

You can get the Power Source from killing the Obsidian Sentinel right at the Uldaman Backdoor(67,44)

As soon as you enter the back instance of Uldaman, he is behind the closed doors.&nbsp The back of Uldaman is at 67,44 in Badlands.

A couple of points that I don't think were very clear from everything above.&nbsp But great information from everyone...

1. AoE spells will not be reflected back at you, but the EFFECTS (stuns, slows) that proc off those spells might be reflected.&nbsp So if I hit him with a Flamestrike, I won't take any damage because I'm not in the AoE, but I MIGHT be stunned if I have that Talent and it procs.&nbsp Same with Blizzard and slowing. That being said, I've never seen him reflect Frost Nova.

2. Reflection happens about 50% of the time, but only on aimed spells.&nbsp CoC can't be reflected, however the damage from Blastwave CAN.&nbsp These are just my observations, feel free to contradict me, but I have combat logs to back this up.

3. The Guardians that splinter from the Sentinel ALSO have the spell reflection property.&nbsp They are level 35, non-elite, and go down pretty quickly. The Obsidian Sentinel is lvl 42 Elite.

4. Sometimes when you leave the instance and return, the Sentinel and his splinter will not have gone back, they will be right there.&nbsp Make sure you go in with Mana shield up, and Frost Nova ready.

5. THE MOST IMPORTANT PART
After he splinters about 4 times, and you have done the run in, run out to heal and mana up, you will now have the long fight in front of you.&nbsp If you have to leave at this point of the fight, you will be resetting all the damage you have done.
With him splintered so many times, he is very small, and does very little damage.

You can stand there with mana shield up, have him hitting you constantly, and your mana regen will be greater than the damage he does to your shield.&nbsp You can sit there and just regen mana if you don't want to produce a new splinter, and just refresh your mana shield every 60 seconds.

Otherwise, you want to be wanding him the whole time until he produces a new splinter.&nbsp At which time, Frost Nova, back off, Flamestrike, CoC, then either Blastwave or Blizzard or wand until the splinter dies.&nbsp

Then go back to mana shield and wanding the Obsidian Sentinel while your mana regens.&nbsp If you need health, next time your Frost Nova is ready nova the Obsidian Sentinel, back off and bandage.

During this fight, his new splinters will NOT reduce the Obsidian Sentinels health any more.&nbsp Once he gets to a certain size he just won't get any smaller or weaker.

GREAT BUFFS TO HAVE
Mighty Trolls' Blood potion, regens health.
Scroll of Spirit to increase your mana regen.
Any stamina buff, any armour buff.
Keep Ice armour up.

NOTES:
When he is splintered down as far as he will go (after about 4 splinters) he has about 8500hp.

If you have a wand that is doing on average 50 per hit (he does resist a fair number of hits), and let's say a 2.0 speed, it will take you about 6 minutes of uninterrupted wanding to take him down.&nbsp However, it IS interrupted each time there is a splinter, so you have closer to 10-15 minutes of just you and the Sentinel to look forward to.

And if you run low on health and have to leave the instance, he will heal up, and you will have to do it ALL over again.&nbsp Once you know he's not getting any smaller, you HAVE to stay the whole fight, dealing with the splinters as they appear.

As mages we're used to very quick, high-DPS-burst- damage kinds of fights. Break that habit quickly, it will get you killed in this fight...and it's a long run from the GY.

Oh, and the last part of the quest where you fight the mana surges is a piece of cake.&nbsp Just stand on the pier and pull them one by one.&nbsp They're ranged, but don't do too much damage.&nbsp Just have Dampen Magic up, and if you need to, stop and drink and eat.&nbsp You've got plenty of time.