The Death and Decay Shell
Deathrite shamanDark ConfidantThoughtseizeAbrupt Decay
This newcomer has rapidly become the backbone of a ton of different fair decks ever since Return to Ravnica was released. What we have here is a base black removal and disruption package that delivers a lot of raw power while covering a lot of different bases as far as interactivity is concerned. Just this shell alone not only gives you mana acceleration, extremely flexible removal for almost any threat you're likely to face, and the ability to interact just about anywhere with discard and graveyard hate but also a lot of raw power in the long game with raw card advantage and Deathrite Shaman's ability to turn used cards into faux Drain Lifes.
Given these strengths, it shouldn't be surprising that this shell has been integrated into and given rise to a large number of strategies. I think the Legacy community even now hasn't finished exploring the potential of this new shell.