Minecraft wiki entry for Things_not_to_do, associated text: If the Wither is spawned in the End, it could create holes in the end stone, increasing the chances of the player falling into the void. If the player spawns the Wither during a fight with the ender dragon, the player has to deal with two bosses. The player may attempt to have the Wither kill the Ender Dragon or distract endermen, but for most players, killing the Ender Dragon is easier without the Wither being there as the Wither often is more of a hassle to the player than the Ender Dragon. So, if the player is planning on fighting the Wither in the End, they should do so after the Ender Dragon is defeated. In this case, battling the Wither could make things easier for the player. Since there are a lot of endermen in the End, the Wither focuses more on attacking the endermen than on attacking the player. Additionally, the endermen retaliate against the Wither, thereby assisting the player in the fight. However, the player should be careful about spawning it near an end city because the Wither flies up the end city to destroy the mobs there, which means the player likely must climb up the end city to reach the Wither. Also, the player should make sure not to land the last hit on the Wither near the edge of an island so that the nether star doesn’t fall into the void.