Deep Spawn 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

Trample At the beginning of your upkeep, sacrifice this creature unless you mill two cards. : This creature gains shroud until end of turn and doesn't untap during your next untap step. Tap this creature. (A creature with shroud can't be the target of spells or abilities.)