The trailer for Clone Wars Season 7 revealed the series would cover the events of Revenge of the Sith, as well as other key elements and scenes from the prequel trilogy. But some major players that didn't appear in the movies may be included as well, marking the official debut of previously apocryphal villains into Star Wars canon.
Reddit user TheDictator26 posted some old, unused Clone Wars concept art from showrunner Dave Filoni of a Yuuzhan Vong creature and scout ship, claiming it was for a scrapped episode that would see the Vong scout ship abduct Jedi for research purposes to determine the strength of the Galactic Empire. Filoni's also working as showrunner for the hugely popular live-action Star Wars show, The Mandalorian, and is widely regarded as a loremaster in the post-George Lucas era of the franchise.
With a new season coming out soon, it's possible the creatures could return in a future episode. This would spell out major bad news for the Clone Wars crew, as the Vong are a treacherous and incredibly deadly species.
The Yuuzhan Vong were an alien species featured in the Extended Universe (now known as "Legends" and not considered canon). They invaded the newly formed New Republic after the events of Return of the Jedi. They exist outside the force, and thus had a potent advantage over the Jedi and nearly defeated them. Their invasion was a bloody one, leading to more than 365 trillion deaths.
The Yuuzhan Vong provide an interesting contrast to the Jedi. While both value religion highly, the Vong center their lives around a philosophy of sacrifice and pain. This philosophy makes them a formidable enemy, as they gladly accept pain and death. While the Jedi combine their Force abilities with droids and spiffy modern machinery, the Yuuzhan Vong reject all technology, instead relying on genetic engineering. This resulted in their use of amphistaffs, which can alter their form, and body-modifying armor.
If the Vong are included in the new season of Clone Wars, it likely won't be to the massive scale depicted in Legends. Instead, we might see a smaller Domain (the Vong word for a clan) in an isolated attack, like in the unproduced episode described in the Reddit post. Still, without the use of many of their Force attacks, it could mean a major threat to the Clone Wars characters, and possibly make way for a more prominent appearance in a later Star Wars story.
Clone Wars Season 7 comes to Disney+ on February 21.