Demilich 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

This spell costs less to cast for each instant and sorcery spell you've cast this turn. Whenever this creature attacks, exile up to one target instant or sorcery card from your graveyard. Copy it. You may cast the copy. You may cast this card from your graveyard by exiling four instant and/or sorcery cards from your graveyard in addition to paying its other costs.