There is only one way to get to the Nether and that is through the Nether portal built with Obsidian. The Nether has a naturally generated biome with a structure that is called the Nether fortress.
An enemy can spawn in the fortress; it is called the blaze. It is a creature made of blaze rods(used for brewing). Other mobs spawn in the Nether like the Enderman, Chicken Jockey, Skeleton, Wither Skeleton, Magma Cubes, Ghast, and Zombie Pigmen.