The Monitor

The Monitor is the supreme leader of the Dominion, a faction of fallen Guardians who believe true power lies not in the Light as promissed by the Traveler, but yes in Darkness. He has been obscessed with conquering the ancient power known as the Anthem. He is also the main antagonist and final boss of Destiny 3's main campaign, Anthem of Creation.

Rebirth from Light
While bit was known about the Monitor's background before he was ressurected - as he rather seems to avoid it - he was known to be ressurected and brought to the Last City as a Guardian. An extremely wise and skilled Guardian, even, so brilliant that he was took by the Cityspeaker as his protége and apprentice personally. The best relationship he ever had was with the Speaker himself, considering he rather seemed extremely reluctant about the behaviour of his own Ghost, Bertrom.

Deception and Exile
The Monitor was constantly haunted by dreams of his ressuraction, in which he describes he was "denied by heaven" and reborn, but not quite feeling alive, as if something in him was missing and it was replaced by the Light that now lies within him. He came to realize Guardians never thought of learning more about the Darkness and its whereabouts, or why it acts in opposition to the Traveler. To answer this question, one day, the Monitor broke into the Speaker's personal studying room, where he found a book written by the Speaker in which the Traveler told his voice about the Anthem of Creation, the force that created Light and Darkness in opposition to balance the universal energies, and that now this balance is being broken, leading to the universe's inevitable demise. Angered by discovering the Cityspeaker has hidden this truth from his people and even the Vanguard, the Monitor was caught by the Guardians and exiled by the Speaker's order for his crimes of invasion against his master.

Following the Darkness
Exiled and left for dead in a space pod, the Monitor was drawn into a black hole. However instead of begging to keep his life, he surrendered, wishing he could finally return to the garden he dreamed of and forget about all the mission instructed to him by the so-called Traveler. However, when entering the singularity, the Monitor entered in contact with the Darkness itself. It has taught him what he saw at the Speaker's books was true, and that if he stead by its side, they would together take control of the Anthem and have their power feed them, but most importantly, the Darkness promissed that with the Anthem, the Monitor would finally feel whole again, as he always dreamed of. Following this, he was warped out of the black hole into Phobos, a moon of Mars, in which he secretly recruited an order of Guardians that felt the same way he did. This army of the Darkness would be called the Dominion, and their sole mission would be to uncover the Traveler and the Speaker's lies, and take possession of the Anthem's power for themselves to be complete.