Secret level in Slither.io doesn't exist. A youtuber named Don't Turn Around filmed himself playing Slither.io and showing us how to get to a secret level. He said he heard about this level on forums, so he wanted to try it out. However, he is the only person who has a proof that this level exists. Many people believe this is edited, and lot of youtubers tried to enter this level but they failed. This leads us to conclusion that secret level doesn't exist. Also, in game-files, there is no such a thing as a 'secret level'.
The instructions the youtuber, Don't Turn Around, said to enter the secret level, your snake must be between 8000-9000 mass. Then you must go east and when you see the border, keep going. The secret tunnel will be revealed. Your snake will go through it, and you will see lot of dead, suicide snakes. In video, we saw a giant snake that killed player. Then, an image of a snake pooped-up. Forums say this snake is supposed to 'hypnotize' you which is not true.