It is in Ghostship, about 104 steps south of the spot the map marks with that name.

And 4 down from the surface once you are there.

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

• Something for minotaur guard, minotaur mage and mummy — they live down there.

The chest is 4 floors under the surface. Going down over it, this is what each floor uses:

1. Ground: down through stairs.
2. 1 down: down through stairs.
3. 2 down: down through stairs.
4. 3 down: down through stairs.

• 1 — the way in: stairs, ground
• → — the treasure lies this way

• 2 — stairs, 1 down
• → — the treasure lies this way

• 3 — stairs, 2 down
• → — the treasure lies this way

• 4 — stairs, 3 down
• → — the treasure lies this way

• 1 — the chest
• 2 — the chest

There are 2 chests here under 2 quest numbers, so you can take them all.