Minecraft wiki entry for Wither, associated text: The wither is spawned by placing 4 blocks of soul sand and/or soul soil in a T shape (see image on the right in the "Behavior" section), and putting 3 wither skeleton skulls on top of the three upper blocks. The last block placed must be one of the three skulls and can be placed by the player or a dispenser. Air blocks are required on either side of the base soul sand or soul soil under the upper blocks (non-solid blocks such as grass and flowers prevent the wither from spawning). The building pattern can have any orientation (including horizontal and upside-down) as well as the skulls; the wither always spawns at the foot of the pattern. When the wither has successfully spawned, it is angered and creates an explosion around itself.