Crystal crab and dundgeon update

Can be included with cave update and the idea is from mowzies mobs modpack. The crystal crab is a rare mob that is fast and can only be damaged by pickaxe. Will drop 1 or 2 diamonds when killed and can burrow away into stone or dirt. A rare dundgeon mob which is like a miniboss. A big suit of armor that attacks with an axe and will only be damaged if you hit the diamond sword in its back while its guard is down. Can break a shield instantly and will drop his helmet and axe. Both on par with diamond or iron and are unbreakable.