"Dweller"

"Dweller" was a permanent agent hidden in Metru Nui, and was monitoring the city from his hidden base in the Archives. He was watching Toa Takanuva after the Matoran returned to Metru Nui, and was tracking him until Order of Mata Nui member Brutaka captured him. Dweller was later killed when Teridax destroyed Daxia

Powers and abilities
"Dweller" could probe the thoughts of others or drive them mad through nightmares. He could also affect others' perceptions to keep them from noticing him, and he was able to spend over a thousand years undetected.