"A coffin marked with the letters D-I-O... I wonder what could be inside?" - Treasure Hunters (Torejāhantā, トレジャーハンター)

Summary:

DIO's Coffin is the spawn point of the NPC DIO, who spawns near here every 60 minutes, adding a 1-minute delay each spawn. Time stops delay it even further. This coffin can be found on one of the map's corners.

In appearance, DIO's coffin is made out of wood with a golden plate. Written on it DIO. Over the plate, there is a door lock made out of gold. In addition to that, there are particles coming out of it. such as a mysterious purple fog. And from the center of it, there're menacing (ゴゴゴゴ) particles, just like the Menacing Dummy.

Trivia:

Near DIO's Coffin, there's a banknote spawner, so while you wait you can grab some of them.
Near DIO's Coffin, there's a Dirt Pile.
As of the latest update, this version of the coffin is much smaller than DIO’s Cave.
With the smaller size, it’ll be harder to put shards or bear traps via The Emperor's or The World: Alternate Universe's / Neo The World: Alternate Universe's abilities.
As of v2.0.1, the entire map and DIO's Coffin has a new look. There is a dirt path on the side of the map that leads to DIO's Coffin.
On the new map the coffin is located next to the hill that Tim has his shop on.
It is a reference to the beginning of Stardust Crusaders where DIO's coffin is risen from the bottom of the sea and opened leading to DIO's return and revival.