A Nether Fortress.

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Nether Fortresses
are the only generated structures in the Nether. They are highly dangerous, but also are the only way to start brewing potions or accessing the End. Nether Fortresses are sometimes found encased in Netherrack, but mostly spawn away from the cliff face over pools of lava. They are made of Nether Brick. A telltale sign of Nether Fortresses is huge, straight columns of dark material poking out of lava

Exploring and Preparation

All of the Nether mobs can spawn here: Ghasts, Blazes, Magma Cubes, Wither Skeletons, Zombie Pigmen, and Skeletons. There are Blaze spawners, while the rest can spawn naturally. Blazes can also spawn in other parts of the fortress. Wither Skeletons (and therefore Skeletons) can only spawn here. When exploring, wield a Bow. This helps you deal with the Blazes. Snowballs also work nicely. Be careful of the seemingly omnipresent Ghasts as always. If you find a 3x3 square of Nether Brick with a lava well in the center, keep going forward. You will find a room full of Soul Sand with Nether Wart on top. The latter is necessary for brewing potions, while the former is necessary for spawning the Wither.

What is obtainable only here