It is in Minoroom, about 12 steps north of the spot the map marks with that name.

And 4 down from the surface once you are there.

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

• Key 4601: a locked door bars the way, and that is the number of the key that opens it.
• A rope. You drop in through a hole and a trapdoor, and a rope is how you climb back out.
• Something for minotaur mage, ghoul and minotaur — 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 a hole.
2. 1 down: down through a trapdoor.
3. 2 down: down through a trapdoor.
4. 3 down: down through a ladder.

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

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

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

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

• 1-3 — the chests
• 4 — Rookgaard Bear Room ON

There are 3 chests here under 3 quest numbers, so you can take them all.