All warlocks gain access to the Doomguard at level 60 with the Curse of Doom spell. If the damage from the curse kills the mob that yields experience or honor, a Doomguard will appear. Unlike the Infernal, the Doomguard does not spawn under the control of the warlock and must be immediately enslaved.
Aside from the Curse of Doom, warlocks also have a class quest to learn the Ritual of Doom, which is a guaranteed way to summon a Doomguard. However, it requires a party of five people to complete the summoning; one of the five will take a large hit to their health pool as a sacrifice. The quests are given by Daio the Decrepit in the Tainted Scar, the southwest area of the Blasted Lands where elite demons reside.
The Prison's Bindings involves killing demons in Dire Maul East for drops. The demons come in groups of 2-4 and groups of up to 3 can be soloed with a little effort by warlocks since they can banish and enslave. Enter DM East through the back door (requires the [Crescent Key]) and reset the instance once you have exhausted manageable groups.
The Prison's Casing requires killing the elite demons in southern Winterspring for drops. These demons have a lot of health (~18k) and hit very hard, so the ideal way to do this quest is with assistance. It is possible to solo them through a combination of chain fearing and Curse of Doom procs, but as it takes a couple of minutes to kill 1 demon and the drop rate is close to 20%, this method is not preferred. It will hurt if a couple of your fears fail.
After turning in both quests, Daio gives the quest Suppression, which asks you to capture a Doomguard Commander from the Tainted Scar with a quest item he provides. Among the demons in the area, the Commanders are found fairly easily, however finding one without Felguard bodyguards or other demonic allies can be difficult. Using the item on the demon is a channeled spell which must be held for 20 seconds. Damage done to the warlock does not interrupt the channel, but the commanders come with a War Stomp ability that will. There a few methods to accomplish this:
Come with a tank and a healer. Have them tank it far enough from you that a war stomp will not affect you.
Have a rogue stunlock.
Have a hunter use a Freezing Trap. Be aware that it only lasts for 20 seconds (untalented) - wait for the traps cooldown and place a second trap between the already-trapped demon and the Warlock channeling the spell.
Begins a ritual that sacrifices a random participant's health to summon a doomguard. Requires the caster and 4 additional party members to complete the ritual. In order to participate, all players must right-click the portal and not move until the ritual is complete.
Curses the target with impending doom, causing X Shadow damage after 1 min. If the target yields experience or honor when it dies from this damage, a Doomguard will be summoned. Cannot be cast on players.
If summoned by Curse of Doom, the demon will arise from the body of the fallen mob as the curse kills it. He is a hostile target and will immediately attack you after coming into being. You must enslave him in order to gain his services, but unlike Ritual of Doom you can theoretically keep him around indefinately.
If summoned by Ritual of Doom, the Doomguard spawns under your control and will stay under your control for 15 minutes, after that time is up it will despawn.
The Doomguard is undoubtedly the most powerful Warlock pet. It's effective use lies in the realisation that the Doomguard is not a minion of brute strength. It must be used skillfully for dealing damage and disabling the enemy so as to excel at PvP. The Doomguard comes with two important abilities that are very powerful when used against any adversary. War Stomp can be used to knock back and stun the enemy, normally enough to cast a shadowbolt or some such similar destructive spell. The Cripple spell may be used to slow down the enemy's movements immensely and is thus ideal for killing melee units, or buying time to perform spells that take casting time. With a combination of these, the Warlock can become an incredibly powerful PvP adversary in the right circumstances.
Remember to cast Curse of Elements on your Doomguard before enslaving it, this will greatly increase the time you can keep him under control. Failure to use the curse can result in initial enslaves that last as little as 5 seconds, and diminishing returns go into effect at that point. The Doomguard is also highly resistant to magic and failure to use the curse can result in a series of resists in which time the demon can easily kill you. The downside is that all his high magic resistance is lost in the act of controlling him. Even when he is under your control, the curse remains on him.
If you Banish the Doomguard a few times when it breaks free, this seems to allow you to control it longer on a re-enslave, apparently following the in-game rules of Diminishing Returns. Use of Curse of Elements will drastically improve your ability to banish him, remember he is very resistant to magic.
If summoned by the use of Curse of Doom, the Doomguard's usual opening move is to cripple you as soon as he's in range, so be ready for him!
Although an unusual glitch, if your party somehow fails to perform Ritual of Doom (for example, clicking on the portal and then moving), you will be unable to recast the spell. There is no cooldown time for this and may remain in that state indefinitely. The known remedies are the warlock doing a "loading zone" change (Examples include: entering/exiting an instance, entering/exiting a battlefield, and switching continents.) or by logging in and out. This glitch does not happen all of the time, and is still under repair.
Doomguards released from enslave or who break free should not be underestimated. They are easily a match for an undergeared/poorly played warlock, hit hard and have even been known to warstomp (stun) mid-cast. Try not to get into risky situations, where enslave breaking could kill you. It's best if you keep him at least 20 yards away at all times.
Letting loose a Doomguard in an area controlled by the enemy faction (particularly starting sections, villages, and cities) will make a VERY enjoyable show; just be careful to make sure that the Doomguard decides to eat the guards and lowbies instead of you!
/Patch 4.0.1 (12-Oct-2010): Abilities removed, summonable alongside another minion new spell added Doom Bolt.
Patch 3.3.0 (08-Dec-2009): This pet now innately have Avoidance like all other warlock pets.
Hotfix (2009-05-13): "The Doomguard's ability Cripple now reduces ranged and melee attack speed by 20% instead of 45%. Cripple's slowing effect does not stack with other slowing effects such as Thunderclap and Infected Wounds."