The ‘inner’ Doom replaces a wall within regular Doom and mimics any movements the player makes.
As Doom continues to be played on just about every piece of technology available, things have finally gone full circle as someone has now made Doom playable in, well, Doom.
As spotted by PCGamesN, YouTube user kgsws uploaded a video of himself modding the game into itself, before showing actual gameplay of him playing Doom within Doom on PC.