Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: Especially if your nether fortress is far from your base, it is nice to have a backup way to get to the Overworld. Bring at least 10 obsidian and a flint and steel. The flint and steel is not necessarily required, since they can be found in chests or a passing blaze or ghast might help you out, but it is good to have a guaranteed portal lighting device. Also remember to have a flint and steel somewhere in the Nether in case your original portal gets blown out.