Fetish Army is a Voodoo Witch Doctor skill unlocked at level 30, which summons group of Fetish demons to fight for the Witch Doctor. This is the Witch Doctor's only summoning skill that generates a large number of minions, albeit non-traditional, short-lived ones.
The fetishes deal substantial damage, making them more than a tanking distraction, though they are handy at that also. The number and variety of Fetish varies, and the skill produces about half a dozen of the little fellows, with a mixture of dagger-wielding Fetishes, blowgun-firing Fetish Hunters, and fire-breathing Fetish Casters. Three of the rune effects increase the number of one type of Fetish, for players who wish to customize the experience.
As of Blizzcon 2009, nothing was known about the skill other than the name and the written description. Some months later a bit more information was added by Bashiok in a March 2010 forum post.
Fetish army is really the only mass-summon spell the witch doctor has. The fetish army spell effects are coming along too, it's looking pretty amazing. But it's really more of an AoE that happens to do its damage by sending out little dudes to fight.
The witch doctor is more about using a small number of summons as damage soaks, and then casting spells for the majority of his damage. Some of which are themselves larger one-off summons. He's not a mass summoner, that's just not the intended flavor of the class.
The base skill can be seen in action in a short video released by Blizzard in early 2012.
Skill tiers were removed shortly before the beta started in September 2011, and Fetish Army became a level 29 skill. The skill system was once again overhauled for the Beta Patch 13 in February 2012, and Fetish Army was classified as a Voodoo skill and moved to level 30.
Fetish Army has, throughout development, been too high-level to play, so there are no gameplay reports describing it.