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Official FFXIII guide
What is the grand scheme of Barthandelus?
Barthandelus is an achitect among fal'Cie, an assigner of roles, who believes that the world has fallen in to ruin since it was created. He also believes that the only way to put things right is to recall the Maker, and he intends to do this through an act of immolation; sacrifice on a grand scale.
And so begins and audacious plan. Cocoon is created; a place to nurture and support human life beyond the natural realm. In contracst to the ferocity and uncertainty of Gran Pulse, Cocoon is a paradise of needs met and desires served. With great ease, humans are drawn out of their ordinary place in the scheme of things to lead lives of pleasure and plenty on the new satellite.
Cocoon is a revealing choice of name. It is a protection, shielding its inhabitants, but also holding them tightly and maintaining them in a juvenile state of development while warding off all outside influences. The larva waits in its cocoon until its day of glory.
Barthandelus intends that rebirth to be a dramatic end to both, because the conclusion of the plan is to sacrifice all life on Cocoon. This is the terrible secret of the place that is home to so many, but the reasoning is brutally simple: with such massive demonstration of death, the Maker must surely respond by returning to his creation.
Why doesn't Barthandelus destroy Cocoon himself?
Powerful as he may seem, Barthandelus is as limited in scope as any other fal'Cie. He must rely on l'Cie to discover the true extent of human potential. As the Analects put it, fal'Cie were made with the Maker's urpose but humans were made with pieces of the divine spirit. The power of a fal'Cie is fixed but humans may attain a much greater power if they pursue it.
Furthermore, Barthandelus simply does not have the means or the strength to achieve his plans directly. That is why he needs to trick others into doing his bidding, and why his plans are founded on lies and deceit. Another translation of the name Barthandelus is "Baldanders", a reference to a mythical creature of many shapes and guises, as would befit the fal'Cie who masquerades as Sanctum hierarch Galenth Dysley. He is a master of illusion, as he demonstrates to the party by imitating Serah and, in the final scenes, by trying to trick Fang into believing that her friends have already turned into Cie'th. It is said that he possesses a rukh familiar called Menrva (referencing the goddess Minerva, or Athena, whose symbol was an owl), but it is possible that this is his true form.
The player may even notice that boss battles with Barthandelus are not rewarded with CP, and that he seems to survive each defeat unscathed, indicating that the party has been fighting a phantom or illusory form each time rather than the real fal'Cie. The secret of Barthandelus is that he possesses no real power of his own, but relies on his ability to manipulate the minds of others. Among the people, he spreads fear and mistrust. His design for the heroes is to inspire despair and hatred, knowing that he can then provoke them into attacking what they hold dearest.
If the heroes don't want to destroy Cocoon, why attack Orphan?
The opening of the Ark and the ensuing rampage of Pulsian beasts continues the part of the plan that began with the Purge. The people will be led to believe that war has broken out. Cocoon has been created as a gigantic sacrificial altar, in which all of the Sanctum fal'Cie have played their part, so the heroes face a dilemma. If they destroy Orphan, they will be playing into the hands of Barthandelus. But if they do nothing, people will still die and the fal'Cie will simply continue to pursue their deranged machinations. While orphan continues to exist, humanity will never be free.
How do Fang and Vanille save Cocoon at the end?
Having undergone all of the tests and trials that Barthandelus placed in their path, the party is strong enough to carry out the final stage of his plan. If just one of them loses hope and becomes Ragnarok, they will have both the power and the furious desire to tear whole worlds apart. And this is exactly what Barthandelus intends.
But with the strength they find in each other, Fang and Vanille make their own choice. Deliberately, they take on the power to become the destroyer of worlds, but use that power in deciding their own fate.
With Orphan defeated and the whole of Cocoon now heading for destruction without its power source, Fang and Vanille assume the form of Ragnarok to dig roots deep into the rock of Gran Pulse. They may be completeing the Focus they were set 500 years ago, but they are now wise to the manipulation of their masters. Creating a massive bridge of matter between the two worlds, they embrace their fate and turn to crystal - and so save the people of Cocoon, supporting the imperiled globe on the crystalline pillar created by their eternal sleep.