Lurking Spinecrawler 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

When Lurking Spinecrawler enters, you may choose a nonland card in your hand. If you do, that card perpetually incorporates and gains "When you cast this spell, each opponent sacrifices a creature. Each opponent who can't discards a card." Whenever you cast a multicolored spell, for each of that spell's colors, each opponent loses 1 life and you gain 1 life.