Negative Zone Portal is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

, : Exile target card from an opponent's graveyard. If it's a creature card, draw a card. At the beginning of your upkeep, if there are four or more creature cards exiled with this artifact, flip a coin. If you lose the flip, sacrifice this artifact and return a card exiled with it at random to its owner's hand.