Ghasts are souls that were stranded in the ether after Shattering. Before the creation of the afterlives by the new gods, there was no way for these souls to be laid to rest, and thus, they were drive insane overtime until they become dark corruptions of their former selves.
A ghast knows nothing but raw instinct and madness as they are in constant pain and agony. By killing other creatures, they ease their pain yet increase their own bloodlust. And during the time period where there was no afterlife, any victims to the ghasts would become ghasts themselves. Ghasts are well-known for utilizing the once extensive portal network that consisted of nebulae, making them nearly impossible to contain until the creation of the afterlife.
Ghasts do not slay their victims directly. They devouring the positive emotions and memories which temporarily eases the ghast's own pain. However, this leaves the victim in a state of irreversible mental illness and depression where they eventually commit suicide or decide to kill others for no discernible reason.
In the interplanar era, ghasts remain one of the main hazards of travelling in the ether as they are capable of boarding etherships and attacking the crew. Thus, when travelling the ether, one should bring light-based weapons to repel them.
The best defense against them was auramancy which temporarily calmed it. Others with more malevolent intentions built dolls for the ghasts to inhabit as a quick means of raising armies to defeat their opponents. It was also not uncommon for governments to execute criminals by locking them up in a prison open for ghast feeding, thus drawing them away from main civilization.
Known as one of the main reasons for the Soul War, ghasts plagued Erudite for the 2,000 years following the Shattering during the time when mortals were freely travelling the universe via portals. Mortalkind, though it had just survived the Shattering, was once again nearing extinction. They desperately called for help to find a long term solution - the new gods were the ones that answered their prayers, and thus, some of the earliest religions saw their birth. The gods agreed to provide salvation and a peaceful death to all those who followed their will.
The Demons, who arrived shortly after, wanted a different solution - to cut off the portal network by destroying the nebulae to isolate the ghast-infested worlds. They would go one by one to reap the ghasts to take them to Brunikor where they could be judged for their sins. Soon, these procedures begun to isolate mortalkind, hampering the spread of the new gods' religions as a side effect. When the approach of the new gods begun to interfere with the Demon's plans, the Soul War began. Quickly realizing a disaster like the Shattering could repeat, hasty, loose truces were strung together to prevent an all-out war. These tensions and conflicts over souls would persist for thousands of years, remaining unresolved to this day. Until the Soul War is resolved, it is likely that ghasts will always exist as an interplanar threat.