I transcribed a bunch of stuff from the lore book about Voidsent a while back. Here's the relevant snippet:
Encyclopaedia Eorzea - Bestiary - Summoning Voidsent (page 296) Wrote:Though one could walk a thousand malms and never reach the other, the void and the Source lie but a hair’s breadth apart, separated by an unseen veil serving to prevent travel between the two worlds. This veil, however, is not absolute, and with the right amount of aetherial force can be torn to create temporary gateways - gateways to which aether-starved voidsent are drawn, sensing the life energies of our realm. Naturally occurring rents are typically minor, allowing only the smallest and weakest of voidsent through before the wound heals. However in the years leading up to and following the Calamity, the barrier between the two planes has grown weak, resulting in larger and more frequently tears and thusly stronger intruders.
Recognising the power that lies in this dark realm, mankind has developed methods of creating artificial rents and therefrom summoning voidsent, binding the beings into service with precarious bloodpacts. In the Third Astral Era, the emperor Xande sought to enter a covenant with the Cloud of Darkness that he might utilise her power in the conquering of the world. Taking cue from the Allagans, mages of Fifth Astral Era Mhach summoned voidsent into Eorzea not only to fight their wars, but to power their creations… an endeavour that ultimately met with disastrous results. Since then, void summoning has fallen out of fashion, being deemed both dangerous and harmful to the world’s aetherial balance. Natheless, this has not prevented cultists from dabbling i nthe deadly art - their achievements finding immortality in forbidden tomes such as the Necrologos.
Voidsent summoning is oft conducted using one of two methods. The first entails the opening of a voidgate - a rent in the veil large enough for most of the realm’s denizens to pass through. However, casting a spell capable not only of opening a hole of sufficient size, but sustaining it for any duration is costly, requiring massive amounts of aether. The second is far more aether-efficient, requiring but a small hole through which the soul or essence of a voidsent passes. The soul will then possess a “vessel†provided by the summoner - the drawback of this method being that, the stronger the voidsent summoned, the more powerful the vessel required to contain the soul.