Soon after Metroid Primes defeat at the hands of Samus and Dark Samus that almost resulted in his destruction by his own "flesh-and-blood", he relocates to a hidden area after reforming himself. From their, he begins to recover and comes to the conclusion that he must do something else the hunters will come after him if they learn of his survival. He finally concludes that he must create a warrior of his own to distract the hunters while he regained his strength and power, and then continue to feed on more phazon so that he may realize his dream of godhood.
Tartarus was thus soon created from phazon, and was named Prime's Champion. He was then given orders to attack the Federation, an action Prime knows will draw the hunters away from him, even though Tartarus will be destroyed. What he doesn't count on is for Tartarus to not only survive but to mutate, something that he will continue to do as he feeds on more phazon.
Upon being created, Tartarus' mind was quite unstable, and he was prone to bouts of destructive rage. His bloodlust was great as well, though his devote loyalty made it so he would follow any command given to him by his maker. As he begins to mutate into stronger, more powerful forms, his mind became more stable, and his personality for sadistic. He soon finds himself craving more power at any cost, yet he is still solely loyal to Prime. When he encounters Dark Samus, he believes that she is the only lifeform worthy of being his "companion", a feeling she does not return. This contradicts Metroid Prime's wishes of her being destroyed, something that starts to causes unease between their master and servant roles.
Throughout the fanfic Tartarus continuously mutates as becomes more and more saturated with phazon energy. Though only five of his forms are encountered, it is hinted that he would continue to mutate only if he came in contact with a massive amount of phazon after reaching his so called "ultimate" form.
Form 1: Tartarus' initial form, his internal structures are highly visible through his transparent skin. Weakly held together by phazon, he aggressively seeks it so that he may feed and become more powerful. His skin is rather soft and weak, and he can be easily damaged by most weapons. He can leech energy in a way similar to a metroid.
Form 2: Now that Tartarus has fed sufficiently on phazon, he now generates the radioactive substance. His skin has now crystallized with phazite, though it is not very thick and still vulnerable to various weaponry. He can now teleport at will and produce blades of phazite that extend from his very own flesh.
Form 3: Tartarus has now grown a thick layer of phazite all over his body, and is now no longer vulnerable to most energy and explosive weapons. He has gained the ability to sprout a whip of phazite capable of absorbing the life energies of his targets.
Form 4: Tartarus now has mutated to the point where he now radiates highly damaging phazon radiation, and his mere touch can cause cellular damage to non phazon-based life forms. Massive growths of phazite have now encompassed his body that can be shot off, and he now can temporarily phase through dimensions in order to avoid attacks.
Form 5: Tartarus ultimate form, this is literally the "alpha and the omega" stage of his evolution. Near invulnerable to almost all weaponry systems, he has no known weaknesses.