The Nether is a realm in Minecraft players can go to. It is basically like the 'hell' of Minecraft: Specifically, a huge, Netherrack composed cavern with magma falls and lava pools. There are also hostile mobs like Ghasts, Zombie Pigmen (They're neutral), Magma Cubes, Blazes, and Wither Skeletons. To enter the Nether you must craft a frame out of obsidian, then light it with Flint and Steel to create a Nether Portal. Ghast Fireballs can destroy portals.
In SMG4's bloopers, the Nether Fortress is guarded by a Creeper-Skined Soldier and an army of Zombie Pigmen orphan kids. It is possible the Soldier is FightingMario54321 in his Minecraft skin. Also, the blooper's idea came from FightingMario54321, so it is strongly evidenced that the Soldier is him.
The nether portal can also go to several other places in SMG4's bloopers.