When Phage the Untouchable enters the battlefield, if you didn't cast it from your hand, you lose the game.
Whenever Phage deals combat damage to a creature, destroy that creature. It can't be regenerated.
Whenever Phage deals combat damage to a player, that player loses the game.
10/4/2004 It is only considered “cast from your hand” if you cast it as a spell from your hand. Putting it onto the battlefield from your hand using a spell or ability will cause you to lose the game.
7/1/2013 In a Commander game where this card is your commander, casting it from the Command zone does not count as casting it from your hand.