Ker
Ker is a Rank S Elite Mark found in Endwalker.
Killing the Elite Mark will reward the player up to 
400 Sacks of Nuts, 
200 Allagan Tomestones of Poetics, 
50 Allagan Tomestones of Aesthetics, 1 
Cracked Anthocluster and 1 
Cracked Dendrocluster.
Coordinates
Elpis - (x22.7, y19.8)
Garlemald - (x19.7, y23.8)
Spawn Conditions
Trigger
Each time a Rank S dies in Endwalker areas, it has a chance to spawn four Ker shroud throughout the zone. Once four of them are killed, Ker will spawn.
If they spawn, the game will announce: "The minions of an extraordinarily powerful mark are on the hunt for prey."
They will withdraw exactly 5 minutes after this message if not engaged, or if disengaged by dying.
When all of them are killed, Ker will spawn.
Time
Ker has no respawn timer. It is randomly decided by the server upon each Endwalker Rank S death whether it will spawn minions or not. Because of this, if the spawning fails because a minion withdraws it has no effect on any timers. It could theoretically spawn minions after every single Rank S death in Endwalker in a row if the game decided to randomly do that.
Tips and Tricks
When it spawns, the game will announce "You sense the presence of a powerful mark..."
- Heliovoid is an untelegraphed column AoE applying a Volnerability Up. Move away from where it's facing.
- Ancient Blizzard III is an untelegraphed donut AoE applying a Vulnerability Up.
- Minax Glare is a Gaze applying Temporary Misdirection, making your every movement in a random direction. Look away.
- Whispered Incantation applies a buff of the same name to Ker which commits the next offensive spell to memory.
- Ancient Holy is a proximity AoE. Move away.
Description
“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 mythological—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...— Basildere, Old Sharlayan Aetheryte Plaza, Old Sharlayan