Howls of the Dead
Start this sidequest in Klysma Quarry.
Drop a coin in the musician's bowl, then speak to him to hear a tale of the restless dead.
Follow your marker up the mountain trail, and you'll meet a man carrying his wife to the Necropolis. Bayek offers to help.
Mount the old man's horse, and ride it all the way up the Necropolis.
Carry the woman's body inside, and set it down where the marker indicates.
Now to track down the ghastly howling. Look for a low passage beneath a flapping red cloth, on a south-facing wall (so you'll be facing north while looking at it).
You'll pass through a chamber strewn with rent limbs, but also one of the location treasures. So, ups and downs.
Continue down the down the passage to find another delusional serial killer. These guys could have formed a club, if you didn't keep killing them. In sequence. As it were.
Despite his claims of divinity, there's nothing noteworthy about him as fighter, and he has no allies. Take him apart.
His death completes the mission.
As a sliver of epilogue, you can speak the old man again, just outside the Necropolis.