Null sighting

Null is the newest in a line of hoaxes created by the Minecraft fanbase. It supposedly leaves signs with the word "null" on them. This is not in any way creepy or unnatural, it's due to a parsing failure by Java. Place down a sign in 1.7, load the world in 1.8, and then load it back in 1.7 again and any unoccupied spaces in your signs will change to "null" (may take a few tries).

Most sightings are inconsistent, though he/she is noted as appearing completely jet black and, unlike other creepypastas, such as Herobrine and The green steve are not moving.

Null's evidence is still invalid, its existence unknown, Mojang did not give an answer about it. But it is possible that this is a flaw in your game, a hacker or a virus that infected your game.