What if Kaz was an Inferni that burned the Reaper's Barge? What if Nina was a normal Ravkan girl with new secrets to hide? What if Matthias discovered he had the powers of the very people his country hated?
Etovost is "thatness", a Grisha theory term that defines qualities that make a thing unique, unlike any other thing in the universe. It is what sets Grisha apart from Otkazat'sya, those blessed with power and those not. This story is the "thatness" to Six of Crows' "thisness", where like calls to slightly altered like.
Etovost is a Grishaverse/Six-of-Crows-specific AU in which every Grisha is now Otkazat'sya, and every Otkazat'sya becomes Grisha.
It is unlike any other thing in the canon Grishaverse. Much will be changed. But there will still be mourners, and there will still be funerals.
DISCLAIMER: All of the characters, settings, and world-building canon are the intellectual property of Leigh Bardugo, the author of the Grishaverse (Shadow and Bone trilogy, Six of Crows duology, and King of Scars duology).