The Fierce Deity

Anyway...

So, yeah, I think Link somehow got ahold on the Tetraforce. Maybe rescuing a certain princess...? He ended up losing control and some evil power took advantage of this. This is why he may be referred to as evil. Link is still in there, but not at the forefront. This is the reason that, while he's still a beast, he helps out Link from Majora's Mask with defeating Majora. He may also hold a personal grudge, which might be why he helps Link. He doesn't go on a freaking murdering spree, like you would expect from him, but is dangerous enough to the point where he terrifies everyone he comes in contact with. He doesn't really take over Link with a strange desire or anything, so the other Link must still be present. Otherwise, I kinda doubt Termina would still be in one piece. He poses more of a threat than the moon and Majora combined, in my opinion.

Also, if you decide to beat all the bosses again using the Fierce Deity Mask, you will find that you can hand them their butts in a matter of minutes, if not seconds. Proving my point that this is a) a freaking powerful force to be reckoned with, b) not just your average mask that can just turn you into a completely different race, like all regular masks everywhere can do, and c) this guy is a freaking beast. Again. He is more powerful than Majora, who is, like, better than Ganon, Vaati, and Demise. Let's just face it. Did either of them go as far as bringing the moon down to the earth? No. Not even close. They wanted power. Majora just wanted destruction. It wanted to play games on a cosmic scale. It already knew it was powerful. An enemy with nothing to lose and all powerful and can kick your butt around just by looking at you is the most dangerous, in my book. Maybe now you can see how powerful the Fierce Deity really is, comparing it like that. He took that and threw it flat on its' face like it was nothing. And laughed. And laughed!!! My god, if you're not seeing it yet, you're mentally blind.