Casting her from your command zone results with a resounding you lose the game, if she enters play. There are several ways to still use her effectively.
I will list a few ways below that will help you to "Make it Work". It is not the strongest style, but black has the tutors to pull it off if you really want to do it. It is funny to see the look on people's face when you reveal Phage as your commander. They don't know whether or not to take you seriously at first, but some might realize how big of a threat you are when you throw down a pair of Lightning Greaves.
A few ways that prevent the loss are (there may be a few others, but this is all I can think of off the top of my head and I can't look through my deck at the moment):
- Having Platinum Angel in play (You can not lose)
- Giving one of your opponents Abyssal Persecutor (keeps their opponents, you, from losing)
- Having a Torpor Orb in play (Phage's enter the battlefield ability does not trigger)
- Having a Sundial of the Infinite in play (End you turn in response to her ability going on the stack
- Having Phage countered by an effect, such as Nether Void or Withering Boon/Dash Hope so she goes to graveyard (choose to let her go to graveyard and use one of black's many ways to get her back into your hand to hard cast from there)