Ok this is a very cool combo that can kill many creatures for only 1 mana, and is even more effective if you have 2 or more engineered plagues. When you play the Engineered Plague(s), choose the same creature type (make sure it isn't a creature type in your deck, you can even make up one yourself, I think). Now, use Unnatural Selection on a creature with health the same or lower than the number of Plagues you have (i.e. you have three in play, so you can kill an anything/3, /2, or /1 ). Also, if your opponent has Engineered Plagues in play of the same creature type, set your creature type to the same (he has 3 Engineered Plagues in play set at Goblin, you play 3 plagues also set at Goblin, now you can kill his 7/6 Scaled Wurm for 1 colorless mana!)