This enchanted ornamental blade alerts you when creatures try to enter your hoard from beneath the ground.
Wondrous Item 3,400 gp
You can plant this blade, usually a dagger or a sword, point-first in the ground. After it has remained there for 24 hours, when any creature burrows to a square within 20 squares of the blade (this distance ignores line of effect), the blade emanates a magical intonation only you can hear that indicates the creature’s location, including depth. You must be within 40 squares of the blade to hear its warning. If the blade is moved, it ceases to function until it is again planted in the ground and stationary for 24 hours.
Published in Dragon Magazine 383, page(s) 45.