What is the fastest MTG Mill deck that can be made?
Opening hand:


Leyline of the Void
Helm of Obedience
2 Dark Rituals
1 Swamp


Turn 1:
Start with Leyline of the Void on the battlefield. Play a Swamp, then a Dark Ritual. Use one black mana to play another Dark Ritual. You will now have BBBBB in your pool. Play the Helm of Obedience and activate it for 1 mana. Your opponent's entire deck will be milled out because none of the cards ever goes to the graveyard.

Had someone use this against me in MTGO once and it took me a minute to work out how it had happened. Takes some serious luck to get this going on Turn 1, but if you can pull it off, it's definitely fun.