The Sledgehammer is capable of destroying any brick in one hit and deal no damage to spiders. It is only obtained by destroying spiders as one of the random item drops. However the spiders will only drop them once the player has reached the lower cavern at a depth of around 2000 blocks-however spiders are rarely found at this depth causing players having to go to the upper cavern at around 800 blocks depth in order to obtain the tool after unlocking it.
It cannot be made at the workbench and a ruby pickaxe or higher is required to be able to reach the bedrock cavern.
The Sledgehammer look like a sledge hammer with a wooden handle and a steel head.