Given that the Master has no tangible form and the Knights are hollow, 'haunted' suits of armor, the Knights may not be individual entities but rather extensions of the Master. The resident of the Citadel may not even be the Master itself, but just a figurehead or extension of a being not shown in-game.

Theories are that it's a god, an alien, an evil spirit, or maybe even a form of ancient artificial intelligence powering itself off of human essence. It is unknown what the Master of the Valley truly is. No one remembers when these events started. The Master has only ever been witnessed by the Boy, and the Chosen have been disappearing for possibly centuries. The hold over the Armored Tricos broken they become disoriented and start falling from the sky and tower as the sun begins to set over the Nest, the evil rule of many years finally broken. Even severed, the tail can still fire lightning, and the Boy uses the Mirror to destroy the Master once and for all. Luckily, the tail is accidentally thrown into the Master's chamber. The valiant beast, however, is overwhelmed, pinned down, and its tail is severed. When one of the beasts attacks the Boy, Trico rushes to defend him. The Master is never seen until very near the end of the game when the Boy and Trico get to the top of the Citadel and discover the Master, and it calls in a large flock of Armored Tricos to deal with the threat. The Knights only slow the two down as they make their way to the Citadel: a massive, white tower in the middle of the Nest where the Master is hidden. However, a lightning storm upsets the Master's plan, breaking its control over one of the beasts], who teams up with the Boy. Once the children are brought to it, they are given over to the Master and consumed to sustain its immortality. In charge of the Nest, the Master sends forth its Armored Tricos to kidnap human children to be brought back, and the Armored Knights capture the few that try to escape within the Nest. No one is sure where the Master came from, or even what it is, but given how long 'the chosen' (human children who are abducted) have been disappearing, one can assume the Master has at least been around for a long time. The Master of the Valley is able to broadcast its control via an antenna-like apparatus atop the Citadel, but does not seem to have any audible manner of speaking. Around the Master's main form is a cage of some kind of metal, and around that is a shield of some kind of dark energy that repels those who come too close.

The Master embodies pure evil and seems to be a stationary orb of blackness covered in green, rune-like markings that resemble the Boy's tattoos.

Tv tropes shadow of the colossus