This block is the block that you get when you "beat the game". Craft it with 4 wither cores which are made from 8 wither stars in a furnace pattern and maybe some more boss related stuff. On each side is the image below and it lets you grab blocks from an inventory like creative mode with tabs. It could be like 1.3.2 without tabs, it just is the ultimate block.

If you want to play creative, change your mode to creative.

D: