If Necropotence is on the battlefield, do cards drawn with the draw step go to the graveyard or get exiled?

Necropotence has a replacement effect that says 'If you would draw a card, exile that card face down instead.' This applies to ALL draws, including the one during your draw step, per CR 614.1a. The card never enters your hand or graveyard — it goes directly to exile.

Additionally, Necropotence has a second relevant ability: 'Whenever you discard a card, exile that card.' So cards that leave your hand via discard are also exiled rather than going to the graveyard. This covers most routes a card might otherwise take to the graveyard while Necropotence is in play.

Replacement effects modify an event before it happens (CR 614.1). Because Necropotence's draw replacement applies, the card goes to exile instead — the draw step draw event is entirely replaced. You do not draw at all in the traditional sense; the card simply moves from library to exile.

Concrete example: It's your draw step. Normally you'd draw one card. With Necropotence on the battlefield, that card is instead exiled face down. At any time you may pay 1 life to put one of those face-down exiled cards into your hand (using Necropotence's activated ability), but it will never touch your graveyard through this process.

Note: If some other effect caused you to put a card from your hand into your graveyard (not a discard), Necropotence's discard trigger would not apply. But for the draw step specifically, exile is always the destination (CR 614.1a, 614.6).