Blight phoenix, Rare, 38g
Burning. Rebirth. Undead.
Attack 20N
N=#Undead in hand
Replace all Undead in Searing fire.
I think it should lose its own Undead keyword. Otherwise, it's too easy to summon it in an Undead deck, and thanks to Necromancy, it's most likely to be summoned into a hand of 7 other Undead cards. I get the feeling people would be able to save their gems until this one specifically comes up. Removing the keyword would mean you'd have to draw it by some other means.
And as it is now, with or without its own Undead keyword, it would be a rather sad draw by the Burning keyword in a Burning deck. I'm not sure what to do about that, however.
Maybe it should have a base amount of damage plus a multiple of N, like Attack: 40 + 10N. That way it's not useless on its own, even if it's still a poor damage/cost ratio for a Rare.