Wow, I didn't know how to customize banners when I tried the newest snapshot. Seems so complicated though :o
Ok I think I got the hang of making banners, but how did you make it so detailed, like his head and the nametag
Nametag is probably an invisible mob (or something else that only redstoners know :p) And the head is... a player skull...?
Nope, you can't see a mob's nametag when the mob is invisible, which kind of makes sense. I instead use WitherSkulls, which when summoned are invisible, and in one of the 1.8 there was a new feature which all entities could have their own nametag. That basiclly made WitherSkulls to have a nametag, even though they are invisible, and that made you possible to make floating text.
Man... Dark prismarine doesn't look too good when its close to you because you see the cracks on it, but from far away it looks like a prefect thing to make a swimming pool's floor from. :D