Phyrexian Infiltrator
2B, Creature Minion 2/2, Invasion Rare
2UU: Exchange control of Phyrexian Infiltrator and target creature.
Don't let this invasion card fool you. If you know the rules well enough, you can completely break this card.  Yes, you can tap 4 mana and give them the infiltrator and take their big nasty.  However, if you have 8 mana, the proper play is as follows:
 
Tap 4 mana, exchange control of infiltrator with your worst creature, ravenous rats, for example.
In response, tap another 4 mana and exchange the Phrexian infiltrator with your opponent's Penumbra Wurm.
 
What happens next? Under Stack rules, the inlfiltrator exchanges with their wurm, then it goes back to you in exchange for the rats.  They get the rats, you get the infiltrator and their creature!
 
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