<p>Both Heibai and Wan Shi Tong turned "dark" in the original series: they both assumed a monstruous form when pissed off. I wish the whole "light = good" and "dark = evil" cliché wasn't done, but it's hardly inconsistent with how it's been depicted. </p><p>What puzzles me is the existence of perpetually dark spirits like Koh and the things Korra encountered in the dark forest, and perpetually light spirits like the dragonbird and Tui and La. </p>

<p>Monstrous? Certainly. <i>Dark</i> per se? Not really. And Hei Bai's control over his form was pretty good, if you think about it; he could briefly assume his dark form when he wanted to caution others, something that suggests at least a little focus on his job. Even Wan Shi Tong managed to keep that eerie yet tasteful shape when he first met Jinora. If he'd actually gone dark, like that bunny, wouldn't he have assumed his wyvern form? </p><p>Frankly, I don't even think Koh <i>can</i> change form; he seems too comfortable the way he is, which is both really creepy and yet somehow soothing. He switches faces to show emotion, he doesn't have to take on a different form. Neither, by the way, does his mother. And even General Old Iron didn't really change either; he just looked a little angrier and only appeared in rainstorms, which might give him a purple halo. </p>

<p>I think simply that some spirits have better control of their dark forms than others. For example, that spirit Bumi played the flute for became calm very quickly just by hearing the song. Keep in mind that the dark spirits thing is framed as being simply a slightly more extreme version of our behaviour under anger and other negative emotions, so it makes sense that some spirits can calm themselves down. </p>

That's just different forms. When they are corrupted by Vaatu they're clearly different.

<p>...so there are different rates for being corrupted? As in, Wan Shi Tong was standing <i>right next to Unalaq</i> yet wasn't corrupted half as fast as Furryfoot in the background? Or at all? While standing next to him as he emanated sheer dark energy? When in Wan's time all the spirits we saw just...magically turned into screaming colourful balls of ectoplasmic combustion? (I have to remember that one.) Also, seriously, what happens to those "purified" spirits? Do they just "become stars" and die like UnaVaatu, or do they reform? So many plot holes in the fabric of the Avatar Reality... I smell a franchise moth! </p>

<p>Ooh, franchise moth. I'll keep that one, too. </p>

<p>It's Lady Tienhai </p>

<p>Wan Shi Tong was already helping him, why would he turn him in a dark spirit? </p>

<p>It's not that he didn't <i>need</i> to be corrupted, it's that by the logic of "malicious emotions can turn spirits into neon psychopaths" he <i>should</i> have turned dark. Unalaq didn't even need to do his water trick on Furryfoot, the rabbit just...changed. But the wise old owl was immune. It's not even that their ideas weren't the same, he was IMMUNE. Franchise moth! </p>

<div class="quote"><i>Avatar Vyakara wrote:
...so there are different rates for being corrupted? As in, Wan Shi Tong was standing <i>right next to Unalaq</i> yet wasn't corrupted half as fast as Furryfoot in the background? Or at all? While standing next to him as he emanated sheer dark energy? When in Wan's time all the spirits we saw just...magically turned into screaming colourful balls of ectoplasmic combustion? (I have to remember that one.) Also, seriously, what happens to those "purified" spirits? Do they just "become stars" and die like UnaVaatu, or do they reform? So many plot holes in the fabric of the Avatar Reality... I smell a franchise moth!</i></div>
<p>Yes, pretty much. Wan Shi Tong is wise and ancient, surely he'd know how to not become dark. Besides, the dragonbird also didn't become dark even as it made physical contact with Unalaq as he actually bent dark energy. </p><p>
</p><p>My personal headcanon is that they simply die. Too much light is bad as well. </p>