This is not a review for the card as a value proposition- as your obligation card, this is the cost for playing Phoenix.
This is possibly the most debilitating obligation card in the game (assuming your gameplan involves being Unleashed, which it probably should); Dark Phoenixis the most powerful Nemesis I've seen, and Consume the World is the only time I've seen a Nemesis deck add a new lose condition. This card makes sure that, instead of the rougly 1/2n, (where n is number of players) chance for you to reveal Shadow of the Past, you have a flat 1/2 chance each villain deck cycle for someone to draw this card and trigger Dark Phoenix, on top of Shadow of the Past. Playing Phoenix means recognizing a roughly 50% chance to deal with Dark Phoenix, assuming the game goes reasonably long, and that's a terrifying proposition.
And to me, that is a feature, not a bug. The "OH NO" moment when I drop a 12 health 2/2 Villainous enemy on the board, and then tell everyone that her scheme is a new lose condition, is honestly a joy. Why do we play Expert mode and buy new villain packs? Because challenge is fun! Having what is basically a second, smaller villain (basically just a second villain in singleplayer) suddenly makes you go into overdrive, makes you panic, reassess your board, reassess the villain, and ask yourself where you should be putting your resources. And that kind of excitement, that kind of panic, that kind of math, is exactly what I love about this game.
(also, if you're building Phoenix offensively, an unleashed phoenix with Combat Training and either Hand Cannon or Godslayer can take out Dark Phoenix in three hits, two if you have another way to get rid of toughness.)
I have comments on a lot of Phoenix's cards, and that's because I honestly love this deck. It is the most high-risk-high-reward hero I've played, and this card is a keystone to that experience.