It is in Shadowthorn, about 18 steps south-west of the spot the map marks with that name.

And 2 down from the surface once you are there.

• 1 — Shadowthorn, as the map itself names it
• 2 — the ground above the treasure

• Something for dragon, demon skeleton and elf arcanist — they live down there.

• 1 — the chest
• 2 — the chest

There are 5 chests here under 5 quest numbers, so you can take them all.