Majin Sonic (Japanese: 魔人ソニック) is a character based on an easter egg in Sonic CD's Sound Test.
Activating his code in Sonic CD shows the infamous FUN IS INFINITE message, with a large group bunch of Majin Sonics layered around each other. All the while, the boss theme plays. The creepiness factor is almost entirely owed to the U.S. OST's boss music, which is much different than the PAL/JPN OST's boss music.
The code to activate it is PCM 12 DA 25.