Minecraft wiki entry for Advancement_guide, associated text: The wither will immediately spawn, but cannot deal any damage nor be done any harm for 10 seconds, until it explodes. At this point, as long as you're within a 100.9 x 100.9 x 103.5 area, you will get the advancement. For more information on the wither, what to expect, what to prepare, and how to fight the wither, see the pages "Wither" and "Tutorials/Defeating the wither".