Clockwork Swarm

Clockwork Swarm enters the battlefield with four +1/+0 counters on it.

Clockwork Swarm can't be blocked by Walls.

At end of combat, if Clockwork Swarm attacked/blocked this combat, remove a +1/+0 counter from it.

[Variable Colorless], [Tap]: Put up to X +1/+0 counters on Clockwork Swarm. This ability can't cause the total number of +1/+0 counters on Clockwork Swarm to be greater than four. Activate this ability only during your upkeep.