The Grave is a block from the OpenBlocks mod. This block is used to save all items in a player's inventory on death. These can be broken with anything, including an empty hand. They will display the player's username on the block. This block has been disabled in most FTB Modpacks containing OpenBlocks due to a bug with Tinkers' Construct where if there are Tinkers' Construct tools in a player's inventory, then there will be no items in the grave. This bug may be fixed at any point. As of version 1.2.8, Graves have an Obsidian-tier blast resistance.
Using the use key with a shovel on a Grave will drop all of the items within its inventory, but without actually breaking the block.