A Vex is part of the Demon world and are the closest things to pure evil in the Caster or Mortal world.. They live in the Abyss.
Vexes were also seen around the fire with Sarafine Duchannes and Abraham Ravenwood channeling the Dark Fire to call the Seventeenth Moon. Abraham used a cast to summon Vexes to cause destruction and death.
It was a formless, dense fog, constantly shifting in a way that gave it shape. It was wrapped in some kind of material like, a shroud or cloak, and it also has no eyes.
Vexes can Travel, disappear and materialize anywhere faster than you can blink. They can also possess the living.
"Saguine effuso, atris diebus,
orietur daemonum legio interfectos ulciscatur.
Si ianua notata inveniri non potuerit,
tellus hiscat ut de terra ipsa ianuam offerat."
"On darkest days, when blood is spilled,
A legion of demons, to avenge those killed.
If a marked door cannot be found,
The earth will open, to offer one from the ground."