Yeah, the Marshal is the Big Bad lurking around in the deep caverns, but it hasn't been introduced in the preview builds yet. This last snapshot maybe offers a hint at how Mojang will transition old worlds to the new depth limit though. 'Grimstone' is a new block that generates just like stone, but from 0ish down to -64ish. My feeling is they'll just turn all current bedrock blocks into grimstone and then just let the world seed create the new stuff underneath. I don't know.
