With a new patch coming (eventually) I've been replaying the questing experience on my alts and the question keeps coming back: something caused all souls to be doomed to the Maw that kicked off this expansion's problems.
1) We see a cutscene of it happening: a large red ball comes through the influx of souls to the Arbiter, incapacitating her. Some incredibly powerful soul?
2) We have some idea of when it did and did not happen: Ysera and Ursoc made it to Ardenweald. It had to have happened after the Emerald Nightmare raid in Legion, but before the Burning of Teldrassil that kicked off BFA, as we rescue night elf souls who burned from the Maw.
What creature would be powerful enough to take out the Arbiter that died in that time period?
Someone who's soul is, oh I don't know, bright red?
Discuss implications/if I'm wrong, because I think I've hit the nail on the head.