If you're a warlock, and you haven't done this quest line, go out and do it. Right now.
Especially if you're a raiding warlock and you have a pesky mage and hunter you want to beat on the DPS meters.
Why? Because a buffed Doomguard does over 1000 DPS. ONE THOUSAND! Thats about 750 more than your average fel hunter (if you're affliction spec'd like me).
And in case people freak out, they should know that Doomguard no longer kills party members. He just does a bit of damage when he's summoned.
The quest line, for a level 80, is really very easy. I only died once, and that was my own doing.
Here's my brief guide:
First, go to Stormwind.
Second, fly out to Nethergarde Keep in the Blasted Lands.
I suppose you could also go to Shattrath, and then fly to the Portal and just go through backwards. Might even be quicker.
So, then you run out to the Tainted Scar, and head north, and go find Daio the Decrepit and get the two quests from him.
One which sends you to Winterspring, and one which sends you to Dire Maul.
Now, Winterspring is straight forward, so I won't go over that beyond, fly there, run south, kill mobs.
Since Dire Maul can be a bit tricky, here's Abs's guide:
You see, what you might not know is that there's a back door to DM, that's due north of Camp Mojache.
You don't have your key, you say? No problem.
Lifetap down as far as you can, and self-suicide with hellfire. Run past the door, and rez on the other side of it.
Walk right in, and you'll find the demon mobs you need to kill right by that door.
Once those two quests are done, you can head back to Tainted Scar, where the same fellow will give you one more follow up quest -- still pretty easy if you're careful, don't go pulling 8 mobs at once.
Then, all you gotta do is be very careful, find one of the guys you need to trap, and do a 10 second channel on him.
Why careful? Because you really don't want to aggro the other mobs around there, they have a ton of HPs and they'll still smoosh ya good.
Anyway, once you get your Doomguard, get some friends to summon it, it lets you do things like this: