Yuuzhan Vong

The Yuuzhan Vong (ooh-ZAHN vawng) are a race of sentients from the Star Wars universe who are notable for causing more destruction and chaos than any other antagonist in the Expanded Universe aside from the Empire and the Sith, and for definitely taking the Most Destructive Species crown. And when you consider exactly how much stuff gets destroyed on a regular basis in the EU, that is not a title to be taken lightly.

They made their first appearance in the New Jedi Order series.

In-Universe
The Yuuzhan Vong (shortened to "Vong" only if you want to be insulting, as using that term alone implies the subject has been cast out of the society) are a cast-system society race whose homeworld was in another galaxy from the Star Wars Galaxy; it was destroyed a long, long time before the long, long time ago, turning them into interstellar nomads. Thousands of years of societal alteration and development produced a society with a masochistic love of pain, death, and sacrifice, a massive aversion to any sort of technological equipment, and one of the most elaborately militarized groups ever to show up in the Star Wars Universe. In other words, trouble.

They became notorious in the Star Wars Universe immediately upon their arrival in the galaxy, an action best compared to gatecrashing a party with loaded guns and all of your friends, and then proceeding to kneecap as many people as possible. They went on a rampage of terraforming and outright brutality that resulted in the bloodiest and nastiest war the Post-Yavin Star Wars Universe had ever seen, which eventually resulted in a pyrrhic victory for the Galactic Alliance, the reformation and slow decline of the New Republic, and the slow re-rise to power of the Empire.

Basically, they took a very delicately balanced scale and kicked it as hard as they could.

In the end, the Yuuzhan Vong wound up more reviled in the Star Wars 'Verse than the Empire in many places (statistically, the Empire was worse; however, the 'Vong came in at an extremely delicate time in the galaxy's history seemingly to do nothing but ruin everyone's fun, and was anti-everyone instead of simply anti-nonhuman), and the Yuuzhan Vong War (actually, Wars, since there were two) wound up completely changing the course of galactic history-- commonly for the worse.

In The PPC
The PPC has made multiple uses of the Yuuzhan Vong's special traits and abilities. For starters, due to their aversion to technology, everything made and used by the Yuuzhan Vong was organic, from their spaceships to their explosives. As a result, the Department of Sufficiently Advanced Technology employs an entire division of Yuuzhan Vong "shapers", having them create and maintain biotic versions of PPC equipment for use by those Agents who, for any number of reasons, can't handle (or aren't allowed to handle/be around) artificially created equipment.

In addition, the Yuuzhan Vong are "entirely outside the Force" (as put by Jacen Solo). That being, they have no Force presence that can be sensed by Jedi, and Force powers like mental suggestion, remote manipulation, and even Force lightning have no effect on them (Force powers can still affect them indirectly-- that is, a rock thrown by the Force will still hit them). For this purpose, the PPC is quite keen on having Yuuzhan Vong Agents working the Star Wars beat-- since so many Star Wars Sues are also Jedi or Sith, their being outside the Force gives them a significant edge up against such Sues, which keeps casualty rates low(er).