The anti-build 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 dissappear after a few moments, making "cheating" (by building blocks to get upwards) rather difficult. Be careful with this block!
The anti-build block is dependant on an anti-builder
block to work (unconfirmed) (see picture).