Zekrom's design is inspired by dragon and electric motifs, featuring a sleek, aerodynamic body with robust, powerful wings and a generator-like tail that harnesses electricity
Underground Sea is never going to cost you two life, while Watery Grave will some percent of the time
Granted, it's at sorcery speed instead of being able to do it on someone else's turn to start off a Storm combo with a bit of a boost, but in all likelihood it's still going to do more for you in a mono-black build than a simple Dark RitualDark Ritual will, for the same mana cost
However the core strategy of the deck, walking the opponent into an expensive play, locking him with Winter Orb, and generating progressive tempo with Quirion Dryad on offense was compelling to the point of irresistibility
On top of that, it also has two additional modes, letting you snipe your opponents graveyards while generating advantage