Minecraft wiki entry for Overworld, associated text: The Overworld is home to a wide variety of mobs. Mobs vary greatly in behavior and the level of danger they present to the player. Different mobs spawn at different times and places depending on the light level and the biome, among other factors. On occasion, a mob can move between the Overworld and another dimension, such as The Nether, by using a portal, but this is uncommon. Boss mobs like the ender dragon and the wither cannot enter portals.