The Coco brothers and, yeah, the MSQ example at the end of 2.0 are the only ones I can think of where a character is possessed and then subsequently freed from that possession.
The MSQ example features an Ascian (not a Voidsent) and involves using an obscene amount of aether - like, an amount available only to the WoL - to damage the possessing being, separating him from his host. But Ascians don't work the same as [classic] Voidsent do. They're a distinct type of being.
The Cocos' example also involves using a large amount of aether, but in this example, it wasn't used against the possessing being (an actual Voidsent this time) - it was used to tempt her. She, a succubus, was occupying the host body (a male lalafell) as an interim, while she gathered the aether necessary to manifest her true form. The trap was thusly: a powerful mage offered himself as tribute. She consumed his aether, consumed the nearby bodies of some women she had already killed, then willingly abandoned the host body to assume her true (more powerful) form. Finally, the Warrior of Light and the other Coco brothers banished the voidsent by defeating her in combat.
The powerful mage in question almost died as a result of her consumption:
His life was only saved by the effects of an experimental mana-amplifying potion, which was originally developed to try and aid those of low aetheric potential in becoming mages (yet fell short of achieving such).
Essentially, the technique allows things to get worse (the voidsent fully manifests, a large quantity of aether is consumed) before it gets better. Unless you can be certain of your capability to defeat the voidsent in question - this one was faced by the Warrior of Light and some of the most highly qualified thaumaturges in Eorzea - it's possible that you might be freeing the person in exchange for unleashing a powerful monster. It's a payoff that's... well, I guess it makes for an interesting question: would you save your friend if it meant endangering other people?
But for a professional voidhunter, as harsh as it sounds, maybe not the favoured technique for dealing with possession. It involves too much risk to the lives of many, simply for the life of one. It involves offering up an amount of aether relative to the power of the voidsent - collecting this is risky in itself. And I can only imagine there's no guarantee that the voidsent will leave the host body unharmed, either - if it's already sustained injuries not compatible with life, or if the Voidsent takes too much aether with it when it goes, then I think the host would perish anyway.
The MSQ example features an Ascian (not a Voidsent) and involves using an obscene amount of aether - like, an amount available only to the WoL - to damage the possessing being, separating him from his host. But Ascians don't work the same as [classic] Voidsent do. They're a distinct type of being.
The Cocos' example also involves using a large amount of aether, but in this example, it wasn't used against the possessing being (an actual Voidsent this time) - it was used to tempt her. She, a succubus, was occupying the host body (a male lalafell) as an interim, while she gathered the aether necessary to manifest her true form. The trap was thusly: a powerful mage offered himself as tribute. She consumed his aether, consumed the nearby bodies of some women she had already killed, then willingly abandoned the host body to assume her true (more powerful) form. Finally, the Warrior of Light and the other Coco brothers banished the voidsent by defeating her in combat.
The powerful mage in question almost died as a result of her consumption:
His life was only saved by the effects of an experimental mana-amplifying potion, which was originally developed to try and aid those of low aetheric potential in becoming mages (yet fell short of achieving such).
Essentially, the technique allows things to get worse (the voidsent fully manifests, a large quantity of aether is consumed) before it gets better. Unless you can be certain of your capability to defeat the voidsent in question - this one was faced by the Warrior of Light and some of the most highly qualified thaumaturges in Eorzea - it's possible that you might be freeing the person in exchange for unleashing a powerful monster. It's a payoff that's... well, I guess it makes for an interesting question: would you save your friend if it meant endangering other people?
But for a professional voidhunter, as harsh as it sounds, maybe not the favoured technique for dealing with possession. It involves too much risk to the lives of many, simply for the life of one. It involves offering up an amount of aether relative to the power of the voidsent - collecting this is risky in itself. And I can only imagine there's no guarantee that the voidsent will leave the host body unharmed, either - if it's already sustained injuries not compatible with life, or if the Voidsent takes too much aether with it when it goes, then I think the host would perish anyway.