Defeating any of the normal S Rank marks has a chance to spawn 4 Ker Shroud Enemies. You get the message The minions of an extraordinarily powerful mark are on the hunt for prey... in your chat box in case they appeared.
The four Ker Shrouds do not drop any rewards, and always appear at the same positions.
Failing to kill just one and resetting it will result in a fail at spawning the SS Mark.
Defeating them will spawn Ker.
Myths serve myriad purposes, but one of the more prominent sorts is cautionary tales. Among such stories, those of Ker may be the most terrifying.
It is said that Ker appears on the battlefield to feast upon the souls of the dead, hunting down the living she lays eyes upon unto the ends of the earth. She brings naught but a swift demise to those who dream of glory.
If I was to postulate that Ker is not merely mythologica - which I do - she must be some manner of ashkin. Undead, in layman's terms. This would explain how millennia can pass without a sighting...
However, legend has it that her "shrouds" are ever in search of new prey for their mistress, so hope yet remains. Should you glimpse these spirits in reddened rags as you sheathe your own bloodied weapon, pray keep your wits about you...