Lady Loki, Agent of Chaos 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

Whenever you cast your first instant, sorcery, or Villain spell each turn, exile it, then exile cards from the top of your library until you exile a nonland card. Lady Loki deals damage to each opponent equal to the difference between that spell's mana value and that nonland card's mana value. You may cast that card without paying its mana cost.