It is in Rookgaard.

And 2 down from the surface once you are there.

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

• Key 4600: a locked door bars the way, and that is the number of the key that opens it.
• A rope. You drop in through a sewer grate, and a rope is how you climb back out.
• Something for ghoul, rotworm and skeleton — they live down there.

The chest is 2 floors under the surface. Going down over it, this is what each floor uses:

1. Ground: down through a sewer grate.
2. 1 down: down through a ladder.

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

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

• 1 — the chest
• 2 — Rookgaard Academy
• 3 — Rookgaard Academy

The strongest 14 of 16 kinds whose nests reach this place.

One chest, one reward.