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.
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.
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.