There will come a point in your Clash of Clans campaign where Gold and Elixir won't be the only two resources you'll have to worry about. Dark Elixir will be the key to really powerful troops.
Currently there are five Dark Elixir troops you can train and recruit, and two extremely powerful Hero units.
Troops
Minion – Attack: Ground & Air / Cost: 6 Dark Elixir / Housing Space: 2
Minions are the most basic of Dark Elixir units. Cheap and dispensible, though they do come with some sweet benefits. They can fly, making ground based weapons useless against them, and Seeking Air Mines can't target them. They're also perfect for setting off Giant Bombs without being affected by them.
However, Wizard Towers are a Minion's worst enemy as they do a ton of splash damage, being able to take out a bunch of Minions at once.
Hog Rider – Attack: Ground / Cost: 40 Dark Elixir / Housing Space: 5
Quite possibly one of the best units offensively, since they can jump over walls. Seriously, any walls! In the later game, you'll notice that a lot of other players will use Hog Riders almost exclusively. The only downside is the amount of Dark Elixir required makes it hard to perform a Barbarian Rush styled attack. Thanks to the Hog Riders exceptional health and strength, they can also be utilized in place of Giants.
Valkyrie – Attack: Ground (Splash) / Cost: 70 Dark Elixir / Housing Space: 11
These giant, two-handed axe wielding maidens do in incredible amount of damage, and are best used against other Hero units. When on the offensive, dropping Valkyries near other Hero units or Clan Castle troops will make quick work of them, thanks to splash damage. They also have high health, making them great tanks.
Valkyries however should never be placed in clumps near the Wizard Tower as they can deal some pretty nasty splash damage to them, and take them out prematurely.
Golem – Attack: Ground / Cost: 450 Dark Elixir / Housing Space: 30
The true tanks of the Dark Elixir Troops. These can soak up a ton of damage, and on top of that, split into two separate Golemites upon death, to continue to do damage until they're both dead. The trade-off to their massive amount of health is their relatively low damage. Golems require high-damage units to assist them in battle in order to be used effectively.
When on the offensive, it's smart to place these down near defenses with splash damage like Wizard Towers and let them soak up the damage, while other units dispose of the attacking tower.
Witch – Attack: Ground & Air (Splash) / Cost: 250 Dark Elixir / Housing Space: 12
The Witch can be thought of more as a Necromancer, as she's able to summon skeletons into battle. She's an extremely powerful unit that has very low hitpoints, and thus is mostly used as a support unit. The best strategy when on the offensive is to have her behind a few tank units, such as Giants or Golems, and let them soak up the damage, while she stays behind and summons skeletons.
Of course her skeletons themselves are rather easily taken out, especially with towers that deal splash damage such as the Wizard Towers, so careful placement of the Witch is key to success.
Heroes
Barbarian King – Attack: Ground / Cost: 10,000 Dark Elixir
Hero units are different in a way that you don't have to keep rebuilding them, instead, when used, they need time to recover and regarin their health. Barbarian King can do a pretty good amount of damage, but does need some support units to make sure he stays alive. The Healer and Archer are two great support units. The Healer will keep his health high, while the Archers can help take out enemies or towers from a distance. The downside is that he can't attack enemies in the air.
Barbarian King is probably best utilized when protecting your own base from an attack. He can be set to Guard, at which point he will attack any troops that come within his vicinity, thus making the placement of his Altar really important.
Archer Queen – Attack: Ranged Ground & Air / Cost: 40,000 Dark Elixir
Though costly, the Archer Queen is a must have unit, as she's extremely powerful, has ranged damage, and can shoot at both Ground and Air enemies. However, Archer Queens aren't that great on their own and can be defeated quite easily. A good tactic is to drop down some tanks such as Giants or Golems, then a row of Archers to start dishing out the damage, and then have the Archer Queen positioned behind them. Placing her down with the Barbarian King is also a good combo as they can cover a lot of ground and deal some considerable damage.
Archer Queen's are best used defensively, since their arrows have a long range, making her a good unit to place in the middle of your base.
