Don't open it upside-down unless you want quite the surprise.
"The Coffin" is a black, alligator-skin briefcase made to look like its namesake. The interior is coated in red, quilted leather. It is quite a bit larger on the inside, appearing to stretch backwards like a dark passageway.
It can only be used to store things commonly found in coffins, such as bodies, clothes, jewelry, photos, weapons, coins, and similar.
There is no cost to use The Coffin. Its storage area is 5x times the normal space that container would have (roughly 100 liters). Objects stored inside weigh their normal weight divided by 5x. Only objects commonly found in coffins can be stored.
Living (or undead) creatures can be stored in your container if appropriate, but they will have access to the items inside and may attempt to break free, destroying the container forever.
This is a unique object that can be lost or stolen.