The Soul Stealer is considered to be the best companion for the Phoenix Soul. The Soul Stealer refers both to the Resonance soul that is able to steal other resonance souls and combine with them, producing more powers for the host, as well as the host of the Soul Stealing Soul.
There are many prophecies and stories regarding the Soul Stealer, but the most common one is that it will become irrevocably drawn toward the Phoenix Soul, and attempt to absorb it.
There have only been three Soul Stealers, and as such is considered a ‘new’ soul. Q is the fourth Soul Stealer. All Soul Stealers aside from the current incarnation have never lived past 16. All Soul Stealers are male (so far).
The first Soul Stealer went mad with power, absorbing all souls it could. It managed to accumulate about thirty Resonance Souls before the host went insane and killed himself.
The second Soul Stealer, having learned from the first, was modest in its Rez use and became good friends with the Phoenix. This Soul Stealer was able to partially absorb the Phoenix, resulting in two hosts with the ability to use fire. It had no ability to regenerate however, and was murdered before it was fully able to absorb the Phoenix. This particular case was why the League was formed.
The third Soul Stealer, instead of absorbing souls into himself, chose to switch souls and hosts. This evoked massed confusion. The Soul Stealer went insane and was murdered, but not before finding out that placing two souls in one body would automatically kill the host, unless one of the souls was not a Resonance Soul(that particular situation resulted in a split personality disorder).
The Soul Stealers main power is to take souls and displace them. Put them into other bodies, or into their own body. The Soul Stealer is limited in the fact that it cannot throw souls anywhere; it cannot bring them to the afterlife nor place them in non-living things or non-sentient beings. While the Soul Stealer can manipulate Souls of humans that have no Rez, it can only manipulate the souls of animals with Rez. (Animals With Rez are particular rare, however).
When the Soul Stealer steals a soul and places it into his own body, the souls merge together. The Host of the Soul Stealer takes on a few personality traits of the host of the soul it has just stolen, as well as the ability to use the previous host’s resonance Soul power. In theory, the Soul Stealer could accumulate unlimited Resonance Souls and use them all at once, however it has never gotten very far, as is documented by the first Soul Stealer.
It was found that if a Soul Stealer absorbed a Resonance Soul into itself, the Soul would not reincarnate with the Soul Stealer. No one knows for sure what happens to the merged souls, but none of them have been seen since they were absorbed, and each new Soul Stealer starts anew with a clean slate, ready to absorb more souls.
One prophecy stated the Soul Stealer is the ultimate enemy to the Phoenix Soul. If the Soul Stealer is able to absorb the Phoenix Soul, and then die, the curse of the Phoenix will be gone forever.
While this prophecy has a belief, it does not hold as large as a following as the prophecy stating the Phoenix will take over the Soul Stealer and commence massive destruction.