He's not just based on Onslaught, he's a straight up spriteswap, right down to the sounds. Didja change any of the coding around?
Also, I dunno how this happened, but when I beat him and he turned everything white and got to the win screen, the win screen was covered in white as well. Same thing happened to the character select screen. I fought him again to turn my game back to normal. I didn't even know that it was possible for a character to flashbang your mugen, but there's a first time for everything :v