The Horus Heresy 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

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.) I — For each opponent, gain control of up to one target nonlegendary creature that player controls for as long as this Saga remains on the battlefield. II — Draw a card for each creature you control but don't own. III — Starting with you, each player chooses a creature. Destroy each creature chosen this way.