The Nightmare is a 1.2 (updated in 1.7) Easy Demon created by Jax (previously GW Jax) and is also the most downloaded and liked Demon level in the game, appearing first on the list when just searching for demons. It was created early in the game (2013), being one of the first five demons in the game. It was later updated, making the level significantly harder and adding extra visual effects. This level is rated Long in terms of length.
The level starts off with quite a basic ball sequence. To pass it, the player must change gravity numerous times while hitting some jump rings or pads. After switching gravity in a four-block wide pass, there are jump pads in which the player must not hit. Before hitting the first one, switch gravity and do the same two more times. The player must hit the final jump pad, and it will bring him/her into a narrow pass. Change gravity along with the way the hills go up and down.
Afterwards the player will hit a anti-gravity cube sequence. This part is basic, as the jumps are not too confusing or fast, and there are no faker rings or pads. In the middle of it, the player will revert back to going right.
Next, there will be another ball sequence. All you will need to do is change gravity to dodge the incoming obstacles. At the end, you must change gravity before entering the portal or you will enter the cube portal in anti-gravity, resulting in crashing.
Right after entering the cube portal, there will be a jump pad and the player will be launched into a ship portal. The ship sequence may be annoying to new players, as you will need to navigate through a stream of pillars made of invisible blocks, with small colored blocks for guidance. You will then enter a cube portal, to fall down a sequence of blocks. The last block has an invisible spike, so the player will need to jump over it. The next ship part following immediately after is only slightly harder, as it is basically the same except you switch gravity in the middle of it.
After falling down another sequence of blocks, you will enter a ship sequence similar to that of Cycles; switching gravity constantly. Following is a cube sequence basically the same as the other two, except that there is a gap in the middle. The next ship sequence, the final one in this pattern, is just flying upside-down through a series of pillars.
After the following cube sequence, there is a ball sequence in which there are numerous gravity portals. These gravity portals, however, are only used to cross gaps. Here, the player must flip gravity whenever there is a gap to avoid crashing.
Following that is another cube sequence that is quite harder than the first one. The times in which you must jump come more rapidly and there is one fake jump ring. Right after is a triple-speed ship sequence, where you will need to fly through a large passageway. The level then ends with a double-speed cube sequence, which requires jumps in a similar pattern as to the end of Polargeist.