"Dweller"

Dweller was a permanent agent hidden in Metru Nui, and was monitoring the city From his hidden base in the Archives. he was waching Toa Takanuva after the Matoran returned to Metru Nui.

it is unknown what "Dweller" did during the events of Dark Mirror and beyond.

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 about a thousand years undetected.