alright youve already seen nui. now take a look at how he looked when he first mutated into a shadow toa.
About this creation
nui was first conrolled by a demon that was with in the darkmatter to make nui's body the perfect vessel for his soul. nui was basicly evil in mindset not knowing he was under evils controll. it wasnt untill kopaku beat him and cast out demon with the enchantments the sages temple had trained him to know. now when nui is on the edge of his life and needs to survive. nui passes out and evil nui comes out but as a personanitly spawned by nui's subconsious.