Phage is a Legendary Creature who costs 3BBBB to cast. Obviously, this is a pretty hard card to cast if you're not in mono-Black. Also, if you cast her from anywhere besides your hand, you lose the game. For a long time this made Phage one of the very few Legendary Creatures (that isn't a flip-card from the Kamigawa block) that you couldn’t reliably play as a Commander. If you try to cast Phage the Untouchable from the Command Zone, you will automatically lose the game…. unless you have a way to Stifle her enter the battlefield ability.
Fortunately, besides cards such as Torpor Orb which cancel out enter the battlefield abilities, there is one way to actually be able to play Phage the Untouchable as a Commander. There’s a land called Command Beacon from Commander 2015. This land allows you to add your Commander to your hand from the Command Zone. While it’s a bit awkward to have this requirement, it’s better than having to play something like Torpor Orb.
Also, Phage has some real benefits once she does become cast from your hand. Whenever Phage deals combat damage to a creature, you immediately destroy that creature, and it can't be regenerated. It's a bit better than Deathtouch, very similar to the ability of Ohran Viper.
But her last ability is the reason that you have to cast her from your hand in order to play her successfully. When she deals combat damage to a player, that player loses the game.
That being said, why wouldn't you want to play her in a mono-Black Commander deck? You certainly can. However, she’s not very popular. Part of this is that reanimating her from your graveyard is out - although returning her to your hand is perfectly fine. Now that I think about it, though, 7 mana isn’t much for a mono-Black deck, and she probably SHOULD see more play than she does. There are ways to make her unblockable after all. (Hint, hint.)
One funny interaction with this card is if your opponent happens to have an effect that has to bring back Phage the Untouchable as a mandatory part of its ability. Rise of the Dark Realms, for example, must bring back all creatures from all graveyards, as does Liliana Vess' ultimate ability. Grave Betrayal, a very popular Commander Enchantment, backfires when Phage dies, killing the player with Grave Betrayal as soon as she hits their side of the board. There's a lot of tricky gimmicks you can play with Phage. So, yeah, she’s worth playing for that reason, although if you play those cards, she can backfire on you, too!
Even with Command Beacon, Phage is still pretty awkward as a Commander, which is kind of sad. Still, she's awesome for a Commander deck built around mono-Black Devotion, such as Erebos, God of the Dead or Mogis, God of Slaughter. Phage has plenty of uses and as long as you build your deck with her major drawback in mind, you could have a pretty sweet secret weapon on hand!