I call BS on this. There is no evidence whatsoever that Fierce Deity has "dark powers" or is going around "corrupting people". None, zilch, nada, the big goose egg. The only power that Fierce Deity use in Majora's Mask is firing magical beams from his sword when Link is at full health (something...
This theory is wrong. The Fierce Deity being "evil" or "demonic" has been debunked so many time it makes me sick (just like the "link is dead in MM" theory or "MM is just a dream or a dreamworld" when Hyrule Historia clearly says it's a parallel universe and we see Skull Kid doing carthwheels...