- Type: Humanoid
- Level: 58-59 (elite)
- Found: Scholomance
The Dark Summoner does not have a lot of health but their power Summon Skeletal Lackey can overwhelm a party. Furthermore, they have two Necrolytes guarding them.
Spells they cast:
Summon Skeletal Lackey
(rarely) Curse of Blood
When they are MCed:
Curse of Blood, Increases physical damage dealt to you for 10 min.
Drain Life, drains 100-101 health per sec for 12 sec.
Strategy
If you have Priests...
Mind Control the Dark Summoner and make him/her fight the other two. Then cut in and attack them. Also, interrupt their summon spells, other than this, there's nothing special.
If you use a Mage...
Simply sheep the Dark Summoner and fight the other two. When fighting the Dark Summoner, use Counterspell, Kick, Shield Bash or Silence to interrupt the Summon Skeletal Lackey spell.
Re. Patch 2.0.1
It should be noted that since the patch, Dark Summoners' in-combat behavior has been noticeably different than before. They now frequently run about ten yards away during the middle of the fight, which can cause a domino effect of additional aggro. Luckily, if your group uses the right strategy, this can be avoided. Simply incapacitate the Dark Summoner using 
[Sap] or sheep, take out the other mobs in the group, then put full DPS on the Summoner at the end. They can't run if they're dead.