It is in Minoroom.

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.
• Key 4603: a locked door bars the way, and that is the number of the key that opens it.
• A rope. You drop in through 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. 1 down: down through a trapdoor.
2. 2 down: down through a trapdoor.
3. 3 down: down through a ladder.

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

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

• 3 — a ladder, 3 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 — Rookgaard Katana Room (1)
• 3 — Rookgaard Bear Room ON

One chest, one reward.