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Amass Zombies X, where X is the number of instant and sorcery cards in your graveyard. (Put X +1/+1 counters on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)
A split card that’s both an instant and a sorcery is counted only once for Invade the City.
Army is a new creature type. It’s possible to control a nontoken Army (perhaps a creature with the changeling ability) and, through combinations of other cards, it’s possible to control multiple Army tokens. When instructed to amass, you may put +1/+1 counters on any of your Army creatures, and you may choose a different one each time.
If there are no instant or sorcery cards in your graveyard, you’ll amass 0. If you don’t control an Army, you’ll create a 0/0 black Zombie Army creature token that will die after Invade the City has finished resolving.
If you don’t control an Army, the Zombie Army token that you create enters the battlefield as a 0/0 creature. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.
Invade the City is still on the stack while you count your instant and sorcery cards in your graveyard. It doesn’t count itself.
The number of instant and sorcery cards in your graveyard is counted only as Invade the City resolves.
