Mono black Deck help?

With that much focus on discarding, you really should have 4 Megrims. But you also have to bear in mind that a lot of decks aren't going to have any reason to hang onto cards onces they have gone throguh their entire hand, so unless they have ays of drawing multiple cards, about half your deck could very well become worthless after turn 4. Usually, decks built around Megrim completely forego creatures (but not creature kills and life gain like Tendrils of Corruptions and Consume Spirit) and also run Howling Mine, Font of Mythos, and Underworld Dreams.