"Shall I speak to the dog in it's own tongue? The rabble that begged me to be their god now shakes their fists at heaven, poisoning the air with empty threats. In what world can YOU face ME? I am EZTLI-TENOCH!"
Eztli-Tenoch is the (self-proclaimed) God-King, who has control over the dead. He is the final boss of the game (if one takes the 'legendary' ending route). The Blood-Mad tribes worship him.
Long ago a King named Sars and his Queen Salea ascended into "Gods" becoming "Eztli-Tenoch" and Tletl-Meztli. Or at least this is what he believed himself to be. His domain included the ancestors of the Nomad.
Eztli-Tenoch was trapped by a blood seal made by Bakal, Zelemir, and Azra's ancestors. Azra's ancestor was a foot-soldier whose last name was was Nefir, and their family was given the position of Royal Librarian for the sacrifice. As Zelemir is Prince Zelemir Ashantimon, it can be assumed that his ancestor was Lord Ashanti. Bakal's ancestor was Lord Ashanti's loyal Vezir, Komnat Kozar.
The Nomad became lost and true to their "Nomadic" nature, breaking up and becoming many different tribes. Only the Blood-Mad tribes of Nomad remembered their King, led by Tletl-Meztli. They dedicated their existence to bringing back Eztli, by continuously spilling blood on the seal to see if it would break it.
Eztli began to awaken in the height of the Quaid-Ash. Zelemir and Bakal spilled their blood and broke the seal in an attempt to gain power enough to end the war. This worked for a time and Zelemir gained a link to Eztli's power. Within a month half the Quaid empire lay in ruins. Zelemir was killed by Bakal, but by then the link with Eztli would not allow Zelemir to die. Without Zelemir, the plague continued.
Zelemir returned years later, as time progressed the Mad tribes began a ruthless search for Azra to break the last seal and free their master. During the process, in the Game+ side quests, Tletl-Meztli's powers joined with Azra, in an attempt to free both her and Ezlti from Godhood and return to being mere mortals, she summons Ezlti. Stating that she has found a way to resist and return to being normal she tries to reason with him, but Eztli merely takes her words as both impossible and a sign of treason. Tletl states that should they ever be reunited, the world would perish, to which Eztli states it would be merely purified.
When Eztli accuses of forgetting who they were Tletl tells Estli it is he who forgotten. She tries to tell hi had they known what they would become, things would have been different, to which Eztli states they would not nor can they go back to being normal people and that they have dreadful responsibilities. Using the world as it stands, Eztli tells he the world has become bad without them, but Tletl tells him it would have been worst with them. When Tletl asks for the gift of destruction to end this, he grants her desires and kills her, proclaiming the last link to his past is gone.
He blames Azra for his loss and tells her to make a choice (this is regards to the two Endings of the game).
If Arza follows Zelemir to stop him creating a new seal, her blood is split, breaking the last part of the seal and freeing Eztli. With the aid of the power of those of the Pit, Azra is given the power to boost herself. With each boost, a piece of Eztli's power is removed. Finally, once defeated he is left in the Spirit World with Zelemir, who uses his link to Eztli to chase after him. Zelemir sacrifices his himself to destroy Zelemir, destorying him in the process.
Part of the power included a form of changing his appearance. Typically, he has the appearance of a man made of golden light. However, in the battle with him, he grows horns and multiple arms. His last form is that of an old man, this could mean that at one point he was human. This idea is furthered with the cutscenes that followed and Tletl-Meztli's side quest in Game+.
Eztli was a an ascended God and had powers over the dead, the Plague being one of those by-products. He could blend corpses together to make new forms such as The Right hand of Eztli.
Azra and Zelemir had a strong blood tie to him due to their "deaths", while Bakal did not. Azra meets her end at the fate due to the Plague, though due to its powers did not truly die. Zelemir was murdered by Bakal while in a trance. Another tie to his power was his own queen Tlentl, although upon her request he granted the end of her life. Since Bakal never saw his own demise, he did not have an attachment to Eztli at all beyond breaking the seal with his own blood. Wrenna also had ties to Eztli as a plague survivor. Owed to her failure to serve the Cosmic Flame, her heart turned cold.
In all cases, once Eztli is defeated, all parties with power loose their abilities while Zelemir lost this and his life.