Pardon me, sir, but I believe you have already been advised once to read the
Card Creation Forum Rules. Kindly review them and update your post to be in compliance with them. Specifically, you need to have a
current wording post in bold text and present some discussion on this card. Why do you think this ability belongs in these colours? Why do you think the mana cost you've given this is appropriate? What makes this card interesting?
For the record, I agree with Gimbles and Vartemis. Nominally, this is cheaper than Engineered Plague, but I believe it does much more. Because it is possible to play this card on turn two without acceleration, you could quickly shut down any tribal deck. Compare that to Engineered Plague, which only shuts down tribes of small creatures and costs three mana to do so. Vartemis has also struck the flavour more soundly than you have. Plagues kill creatures. They don't prevent them from being born, as it were. (Techniclaly, Gimbles's suggestion kills creatures too, although it retains the flavour of preventing them from being born. I would take that idea even a step further. In Magic, plagues often kill by giving toughness penalties. My rendition of this card would look be play up that theme.
Plague Mage


{B}
Creature -- Rat Wizard
1/1
When Plague Mage comes into play, choose a creature type.

: Target creature of the chosen type gets -1/-1 until end of turn.

, Sacrifice Plague Mage: Destroy target creature of the chosen type.
1. This card is mono-black. I don't really see any reason why this card should be green. I don't see any green influence in the mechanic you have designed.
2. I've given this a race/class designation instead of a class/class designation. If this wizard is even partly black, there really isn't anything rogue about spreading plague.
3. I've incorporated aspects both of killing outright and of spreading plague. The abilities are slightly at odds with one another, because they both include tapping. I did this for a reason. If you don't have to tap to activate the destruction ability, then this essentially becomes a 3-cc sorcery that reads "Destroy target creature." I did not want to increase the mana cost, so I included the

.