The Anti-built Block is a technical block that can be found in Ur-Ghast towers which ensures that every broken block in its radius (5-10 blocks) will be replaced by a copy of itself. Another function is that every placed block in its radius will disappear after a few moments, making "cheating" (by building blocks to get upwards) rather difficult. Be careful with this block!
The anti-built block is dependent on an anti-builder to be created, but can be mined with silk touch to drop itself which can be placed elsewhere.