Sabb, java is hundred times easier than LUA, I've already made a mod for minecraft but dropped it in the middle do to MCP being updated too fast anyway.
I can help you with the skin of the mob but coding mobs is not to me, I can code items and blocks...
I can try and find the tutorial I learned from and PM you it.
Altho' I need to know the idea itself, the idea of the monster going into the block might be tricky and you'll need to be at least mid to good in minecraft Java.
but just making a mob and the other stuff is rather easy.