Trap Card
This card would be similar to a blockade except it would be trigger by the enemy invading the territory where the trap card would be played. Could also be called a Mine Card.
The enemy would invade a territory and those armies would turn into a large neutral stack, like a blockade. It would be defensive in that in an enemy took a bottleneck territory you'd effectively give up that bonus. Or as an offensive weapon, you break an enemy's bonus, drop the trap card, and when they try and take it back, it turns into a neutral. (this could be mitigated if there was perhaps a "turn timer" to activate the card, to discourage or limit the use of the card as a offensive weapon.
Completed in mod "Custom Card Package 2"