A series like Avatar: The Last Airbender seems to encourage speculation from its avid fan base, and its sequel series The Legend of Korra only further fueled those flames. One of the things fans have spent the longest time speculating about is the possibility of a Dark Avatar -- a force of evil whose elemental prowess could rival even the Avatar's own skills.
However, the truth is that the franchise has not only had that once before -- it's actually had it twice. And since the Avatar spirit reincarnates into every generation, a future Avatar could be forced to face both Dark Avatars to test the limits of their power.
Much of the lore informing how the spirits of Avatar work traces back to the second season of The Legend of Korra. There, fans first learned of the Avatar's origins as a bender who fused with an ancient spirit named Raava. After collecting the ability to bend all four elements, Raava's spirit persisted across future generations as the Avatar reincarnated into a new form. Infused with so much cosmic power, a fully-realized Avatar had abilities so strong that no mortal could rival it. That is why, in presenting meaningful challenges, the franchise introduced two different benders who fused with spirits that could test the Avatar's strength.
Perhaps the best known of these Dark Avatars was Unalaq, who fused with Raava's rival Vaatu in Korra's season two finale. The excess spiritual energy allowed Unalaq's waterbending to ascend to unparalleled heights that rivaled even Korra's mastery of all four elements. At the end of the season, Korra managed to use her spiritbending to dissipate the fused being known as UnaVaatu, although that same season's insistence that spirits could not be destroyed always left open the possibility he could return in a reincarnated form. It stands to reason that the same possibility would remain for an even earlier human/spirit fusion known as Yun.
Featured as the primary villain in The Shadow of Kyoshi, the earthbending prodigy Yun was once Avatar Kyoshi's dear friend before his fusion with the ancient spirit Father Glowworm. When Yun's acumen combined with Glowworm's power, he became a threat that could not only overwhelm Kyoshi head-on but even overpower her friends fighting alongside her. By the novel's end, Kyoshi was forced to put her friend to rest, leaving yet again the open possibility of his reincarnated return, much as with UnaVaatu.
What proves interesting about both cases of a Dark Avatar is that the hybrid human/spirit fusions did not grant the benders the ability to bend any additional elements. While it seemingly boosted the level of power they could produce, allowing Unalaq to ride massive columns of water and split boulders with streams of water, and Yun to devastate entire estates with waves of earth, they never truly felt like a Dark Avatar because they did not mimic the Avatar's ability to bend more than one element. While both could return in the future, combining their abilities that even alone would rival the Avatar, there is the far more intriguing possibility that further fusions with spirits could recreate the mastery of all four elements like the Avatar.
Such malicious forces are already known to exist, with both Koh the Face Stealer and the giant owl Wan Shi Tong known to hold grudges against the Avatar. Alongside Vaatu and Father Glowworm, there could finally be a true Dark Avatar capable not only of bending all four elements, but doing so with more raw cosmic power than the series has ever seen before.
The possibilities are certainly endless. As of now, there is little indication what direction Avatar Studios hopes to take the future of the franchise, but it's hard to ignore that such possibilities remain entirely on the table.
