It is in FibulaDungeon, about 78 steps south-east of the spot the map marks with that name.

And 3 down from the surface once you are there.

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

• A rope. You drop in through a trapdoor, and a rope is how you climb back out.
• Something for dragon lord, dragon and demon skeleton — they live down there.

The chest is 3 floors under the surface. Going down over it, this is what each floor uses:

1. 2 down: down through a trapdoor.

• 1 — the way in: a trapdoor, 2 down
• → — the treasure lies this way

Some floors are not on the list: their way down is further off than the rest, and that stretch you have to find yourself.

• 1 — the chest
• 2 — the chest
• 3 — the chest
• 4 — the chest
• 5 — the chest
• 6 — Fibula Dungeon Exit
• 7 — Deeper Fibula Entrance
• 8 — Deeper Fibula Exit

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