The Book of Makhzor

In the final days of the rule of Emperor Keine Raam, he foresaw that
he would be supplanted, and enacted a ritual that he had been
developing. Through this ritual he brought to our universe an
immortal creature called “Makhzor”, intending to bind it to his
will and use it to stave off those who would claim his throne. The
ritual, however, drained him. As Keine went into hiding to recover,
one of his subordinates (then-Dark Councilor Willerick, now Lord Emperor Arestenax) climbed to the throne and restored order to the
headless Imperium. Given that he had been summoned to serve the
Imperium, Makhzor (now free to do as he wished) chose to follow
Raam’s successor and the Imperium overall.
Makhzor was a being known as an Old Soul, and their (and his) story is as follows.
In the later
years of their galactic rule, the seemingly all-powerful Celestials
had been experimenting with the Force. They were aware that they
would be departing soon, and sought to place fail-safes in order to
continue their work of maintaining the Balance. Leaving behind their
servant races and the Tho Yors which would lead to the founding of the Je'daii, they nevertheless worried that these
would not be enough. Mortal beings can be corrupted and killed, and
may damage the balance more than they aid it.
Their solution
was the Guardian. An artificial Force Spirit, the Guardian was
immortal and immensely powerful, programmed to seek only one thing:
Balance. However, in their wisdom they saw that this Guardian, too, was flawed – too great and mighty, and without empathy of any sort
for the mortal creatures it was to rule. Disappointed in their last
work, the Celestials departed without activating the Guardian, and
for millennia it slept.
It was awakened
by the Darkness brought by the Rakatan Infinite Empire at its height.
A cruel and greedy race, with technology powered by slaves forced to
embrace the Dark Side, they spit in the face of everything the
Guardian was created to serve. Attuned to the Force and feeling the
imbalance the Rakata caused, it forced itself to consciousness. Seeing the
devastation that blossomed in their wake, and knowing that its one and only purpose
was at risk, the Guardian lashed out in rage and fear. 
The Guardian
produced a plague, carried by the Force, which would target only
those Force-signatures that belonged to Rakata. It believed that this
would wipe out the race quickly enough and would cease the spread of
Darkness. It was wrong. The panic brought on by an inexplicable and
incurable plague only pushed the Rakata further towards the Dark,
expanding their borders faster than ever before in an effort to find
a way to save their race. When this seemed impossible, they began to
turn upon themselves, each blaming the other for this tragedy.
The Rakata’s
organized Darkness quickly became chaotic, and the Balance was
further tipped away from the Light. The Guardian realized its mistake
and, unsure how to correct it, simply cut the remaining Rakata off
from the Force. This made them immune to the Force-plague, and
stopped them from further utilizing their Dark Side technology. 
The Guardian
realized that it had acted in error and that direct interference was
too dangerous, given its power. To aid it, it began to create
smaller, weaker versions of itself. These smaller Guardians would act
to bring balance only through
the manipulation of mortals.
In
a very short amount of time, these smaller Guardians, later known to
those few mortals aware of their presence as “Old Souls”, split
into two subcategories. The first and most common was the Bakar Old
Soul. These beings would possess a mortal infant who had the
potential to aid in the quest for Balance. Dominating the body’s
mind from birth, the Old Soul prevented it from developing its own
consciousness and thus used it as a puppet in its mission. At the
death of the body, the Bakar Old Soul absorbed the body’s life
energy and fed on it. 
The
second type of Old Soul was called a Ro’a. The Ro’a had a
significantly smaller population, and worked towards the balance in a
very different way. Instead of possessing a child at birth, the Ro’a
remained bodiless and free. They did, however, bind themselves to a
mortal at some point in the mortal’s life, granting said creature a
far stronger connection to the Force than it would otherwise have
had. This connection provided
more frequent and more vivid Force-visions, which the Ro’a
manipulated in order to guide the mortal in the quest for Balance. With
each vision it provided, the Ro’a in return took a small portion
of the mortal’s life energy.
When
an Old Soul absorbed enough life energy, it tore off a piece of its
essence in a violent and painful display of asexual reproduction.
This new Old Soul would have many of its originator’s memories and
thoughts, but would differ significantly in personality. One thing
that was almost always certain was that a fresh Old Soul would choose
the method (Ro’a or Bakar) that its originator used. The Ro’a
method of absorbing energy was far slower and less efficient than the
Bakar method, so the Bakar reproduced more often, which is why they
made up most of the Old Soul population.
Makhzor
was a ten thousand year-old Bakar Old Soul, and had lived through the
lives of countless mortals. He had not quite finished his last
mortal life when Keine Raam’s ritual brought him from his parallel
universe, overrun with Old Souls (a
universe he called Roash), to
this one, that of Arestenax and the Imperium (which
he called Bet). Makhzor
speculated for many years about the existence and form of the
Multiverse, but did not know for certain until he obtained the Right of the First.
Upon his arrival to Bet, Makhzor researched galactic history in order to discover the differences between
Roash and Bet. Not only was he in the
relative past for his timeline by some thirteen thousand years, but
history after the fall of the Infinite Empire was significantly
different. After years of
further research, Makhzor
traveled to Hoth and there found the
Guardian of Bet. Once revealed, the Guardian explained the divergence in history to Makhzor. 
While
the Guardian of Roash decided not to interfere
directly after it destroyed the Infinite Empire, the Guardian of Bet
decided not to interfere at all. The Guardian of Roash created Old
Souls, while the Guardian of Bet hid itself away and ignored its
purpose entirely. Makhzor chastised the Guardian of Bet for
abandoning its mission and hiding away from that which it was meant
to protect. The Guardian taught Makhzor, though, that it could
observe the multiverse and taught him its form so that Makhzor would
understand. 
The
Multiverse was shaped much like a solar system. Each universe was a
planet in orbit around a common center, which the Guardian knew as
“the Source”. The Source radiated Force energy like a star
radiates heat and light, and was where the Force originated. The goal
of the Balance sought by the Celestials, then the Je’daii, then the
Guardians, was to bring a universe back to the Source. Once balance
was achieved, a universe would tighten its orbit and slowly descend
into the Source, granting all inhabitants unity, power, knowledge,
and wisdom beyond anyone’s wildest dreams.
The
Guardian showed this image to Makhzor in his mind, and then
demonstrated the effects of Old Soul reproduction. Each time a new
Old Soul was formed, the universe in which it lived moved subtly but
distinctly further from the Source. Old Souls, in their quest for
balance, were killing their universe by pushing it further from that
which granted life. Not only that, but Guardians were tied to the
Source and would not move beyond a certain distance from it. By the time
the Guardian of Roash realized what was happening, it was too late,
and it was forced from its universe and into the void between
realities.
Makhzor
lamented these losses, but did not believe this to be sufficient
excuse to not work towards Balance. The Guardian argued with Makhzor,
eventually planting in his mind the seed of an idea. Acting on this,
Mazkhor demanded that the Guardian give up its power and wisdom so
that he else might use them for their intended purpose. 
The
Guardian granted Makhzor his demand, bestowing on him the Right of
the First, and only then did Makhzor see that the entire interaction
between himself and the Guardian had been planned, down to the
letter, by the Guardian. It had known Makhzor’s words and actions
before he had even been brought to this universe, and Makhzor
understood the power he had been granted and the risks it entailed.
The
Guardian, in that conversation, had purposefully deviated from its
preordained course and given Makhzor the idea of taking its power
earlier than he would have come up with it on his own. This had significantly changed events and even shifted the
course of the universe slightly, though insignificantly. Thus Makhzor
learned his first lesson after being granted the Right of the First,
and made it clear to the Emperor he served that he could not deviate
from the path he was on, at least not significantly.
Makhzor’s
purpose remained Balance, however, and he worked towards it just as
hard as he had before obtaining the Right of the First. Towards this
end, he served the Imperium, intending to unite the Dark Side beneath
its banner in order to better fight a united Light Side,
engaging in eternal war and thus bringing Balance.
Years
passed, and the Sith Emperor Vitiate made his purpose clear. Though
it was later realized that, in universe Bet, Vitiate (later known as Valkorion) did not
actively seek the extermination of all life, this was not so in all
universes. Makhzor observed many universes winking out from
existence, and ascertained that various versions of Vitiate were to
blame. The number of universes being affected was growing
exponentially, and with every one that ceased to exist, the Source
became weaker. 
Makhzor
went to those universes being affected which he could reach, stopping
the Emperor there, but the Right of the First had bound him
to the Source and he was unable to move far from it. Many universes were dying
that were out of his range. In a last desperate attempt to save the
Source, Makhzor split his being into many Shards, sending these
Shards to various affected universes. He cannot return, and is
considered dead but victorious. He left behind Bahr've'ahz to continue his work of helping the Imperium unite the Dark Side.