The Ur-Dragon Dragon ambush We like Scion of the Ur-Dragon a lot, but if you dont mind ramping up a bit more, The Ur-Dragon is incredibly mighty as a MTG commander
There are lots of cards that generate card draw with dying creatures but not so many that give you mana
Short version: its a ready-to-play 100-card Commander deck, mono-black, built around artifacts and recursion
This horde of creatures, which amasses more quickly than you might expect, is what you'll use to win the game
If you cast a spell for which mana may be spent as though it were mana of any color, you may cast it for Leyline of Mutation's second ability's alternative cost and still spend mana as though it were mana of any color
For example, during a Bloomburrow Limited event, you can't add Rat Colonies from your personal collection