Sentry Bot 
Artifact Creature — Robot
Flash
This spell costs 
less to cast for each creature attacking you.
When this creature enters, you get 
for each creature attacking you.
At the beginning of combat on your turn, you may pay . If you do, put a +1/+1 counter on each creature you control.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL