When Skullwinder enters the battlefield, it triggers, returning Snap from your graveyard to your hand, and causing an opponent to return any card from their graveyard to their hand.
Repeat.
Results
Infinite colored mana.
Infinite ETB.
Infinite LTB.
Infinite magecraft triggers.
Infinite storm count.
Infinite untap of lands you control.
Target opponent returns all cards in their graveyard to their hand.