A favorite from the Warcraft RTS games, this guy is partly responsible for many players selecting their first warlock. He appeared on the original WoW cinematic looking all 'ard and then climbing a tower to eat a mage. Those who quest in Desolace in their 30s encounter them in a pleasantly demonic zone. We also find them causing much disruption when we first arrive in Shadowmoon Valley. Strangely though, many locks don't bother to do the quest to summon their own. I expect this is related to their general lack of utility, but it probably also has something to do with being available at the same time as the felguard talent.
Level 50 is a busy time for warlocks; you have your mind on changing talent build to get the felguard (as recommended by typical leveling guides), you have your Sunken Temple quest to think about (always worth doing at 50) and you have the infernal to learn to summon. Luckily for us the troll and infernal quests come from the same awesome little imp. Impsy can be found stood beside his less interesting 'master' (Niby the Almighty) in Felwood, just North of the Bloodvenom Falls. In case you were wondering, yes, Niby does make a return in Icecrown Citadel, where he will sell you your tier 10 gear.
The Infernal quest line starts with Niby but all he does is tell you to talk to his imp. Impsy will in turn send you off to gather three items that drop from demons in Felwood, Azshara and The Blasted Lands. He'll turn these into a Fel Fire with which you can reignite Kroshius, then you nip off to resurrect and subsequently kill Kroshius. This shouldn't be too tricky as he's no longer an elite, he used to be pretty hard to solo or would require a group to kill. Bring back his core, watch a funny skit involving a chicken and receive an off-hand item and the ability to cast Inferno. You can actually learn the spell from a book that drops in Blackrock Spire but the drop rate isn't great and why would you want to avoid visiting Impsy?
Inferno is one of the few warlock spells that require a reagent (other than a soul shard) so make sure you keep a few of these if you want to cast it. Summoning the infernal is different to your normal minions, rather than casting a long spell you aim a reticule on the ground at which point a meteor will fall from the sky, stun those around and from the crater will rise your infernal. He's a pretty unsubtle fellow who has no special abilities to use and simply clomps up to things and hits them hard. He does have a passive immolation aura and complete immunity to fire spells which can be quite nice in the right situation (should you find yourself surrounded by lots of little guys casting fire spells for example).
After one minute your infernal will simply crumble to rubble. Historically he was only enslaved and so when his time came he would break free and attack you or anyone else that annoyed him; this is where he got his bad name.
There is actually two ways to summon a Doomguard. One is to kill something with Curse of Doom, though it has to be something you would get experience for. The easiest things to do this on are elementals and demons as you can cast CoD then Banish them, followed by a rank one Banish; that should keep them busy until the CoD pops. A Doomguard summoned in this way is free and will attack you so make sure you are ready to Enslave it.
The other way to summon a Doomguard is through the Ritual of Doom. This also needs a reagent as well as four other people to click the portal. In days gone by the ritual would kill one of the people performing it (no durability loss) but for some reason this was seen to be a bad thing. Now one person only loses some health, so no fun there. Once summoned in this way he is automatically enslaved for 15 minutes with no chance of breaking free. After that time he just despawns.
The doomguard is a pretty powerful demon and as such has been used a fair bit by affliction locks. The reason only afflocks have used him is simply because until recently (patch 3.3) they don't get any real benefit from their minion. Demonology locks will lose way too much DPS without their felguard and the same is true of destruction and their imps. Now that felpuppies buff affliction locks nicely the Doomguard has no place in raids once again.
The ritual of Doom spell can also be learned from a book but the drop rate is still not too good, I would recommend the quest chain that will also teach it. You need to go and see Daio the Decrepit in the North of the Tainted Scar (SW Blasted Lands), and pick up the quests The Prison's Casing and The Prison's Bindings. The first will require killing mobs in the southern part of Winterspring; this will be easy at 80 but at 60 (when the quest is available) I strongly recommend you take a friend or two. The second quest requires a trip to Dire Maul, again, at 80 this is easy enough but at 60 you will want a group. The easiest (quickest) way to the mobs you need in DiM is through the back door in the pavilion with the sad elf. Once you have done these quests you can do Suppression which again is a group thing at 60. These quests are steeped in warlock-juice and so great fun to do.
The doomguard himself has three abilities: War Stomp, Cripple and Rain of Fire. They're all pretty useful though he doesn't like using them much himself so you might need to command him to use them when appropriate. He also hits pretty hard and has a massive health and mana pool. He sounds really useful but frankly, with the effort required to summon him, the massive cooldown between uses and the fact that you can't have any other (more useful) minion while he is out makes him a novelty demon.
Both these guys are, to my mind, in dire need of a revamp for Cataclysm. It seems to me that they were introduced more from a role-play "wouldn't this be cool" angle than from any serious attempt to add something to the warlock class they would use. The Doomguard utility in early Wrath raiding was more a failure of any other minion to be worth using rather than any great strength of its own and the cooldown meant that you could only use him some of the time. The big problem they suffer from is not benefiting from any of the lock talents. In this respect they behave more like guardians than minions. This isn't a terrible thing in its own right but if they aren't going to behave like our minions then I don't think they should replace them.
I would like to see these guys used more like the shaman fire elementals. Those moments when you just want to get out a bit more burst, maybe you're looking at a 1% wipe or a group of adds has just broken loose (many whelps...). Your minion should still be doing their thing but for a short while you call in the big guns. Not something to use every fight, that would become a boring 'rotation' thing, but just a little something extra now and then. Failing that, you should certainly have your minion returned to you when they despawn and not have to resummon them again. That way the infernal and doomguard could be used mid-fight.
As I've said before, I would love to see warlock minions become more diversely used; a minion for a job, not for a talent build. We have learned to reach through into the nether and pull forth diverse demons to do our bidding, demons that are specialists in different situations; it's such a shame to only use one for every job.
Blood Pact is a weekly column detailing DoTs, demons, and all the dastardly deeds done by Warlocks. If you're curious about what's new with Locks since the last patch, check out WoW.com's guide to patch 3.3 or find out what's upcoming in Cataclysm from the BlizzCon 2009: Class Discussion Panel.