Minecraft wiki entry for End_Crystal, associated text: As items, end crystals may be placed on bedrock and obsidian, if the two blocks above the bedrock or obsidian block are air or replaceable blocks and no other entities intersect the area. If four are placed on the end exit portal, one on each of the flat sides, the crystals respawn the original end crystals on the obsidian pillars, as well as resurrect the dragon itself, before exploding. The top of each pillar also explodes, destroying any player-placed blocks. This happens even if TNT explosions are turned off in settings.