High King Kenrith's armies followed him into the afterlife, ready to charge at his command.

Moonshaker Cavalry

Flying
When this creature enters, creatures you control gain flying and get +X/+X until end of turn, where X is the number of creatures you control.

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Wilds of Eldraine

Flying
When Moonshaker Cavalry enters the battlefield, creatures you control gain flying and get +X/+X until end of turn, where X is the number of creatures you control.

Wilds of Eldraine Promos

Moonshaker Cavalry's triggered ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't be affected.

The value of X is determined only as Moonshaker Cavalry's triggered ability resolves, potentially including Moonshaker Cavalry itself. It won't change later in the turn if the number of creatures you control changes.