Sentry Bot 

Artifact Creature — Robot
Flash
This spell costs {1} less to cast for each creature attacking you.
When this creature enters, you get {E} for each creature attacking you.
At the beginning of combat on your turn, you may pay {E}{E}{E}. If you do, put a +1/+1 counter on each creature you control.
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Cards that do something similar expected a card that isn't here?
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Static Snare 92%
$0.14#16022
"This spell costs {1} less to cast for each attacking creature."
+1 more matching line
Ancient Stone Idol 92%
$1.14#2496
"This spell costs {1} less to cast for each attacking creature."
+1 more matching line
Search Party Captain 86%
$0.16#18550
"This spell costs {1} less to cast for each creature you attacked with this turn."
The Mary Janes 85%
$0.32#22057
"This spell costs {1} less to cast for each creature that attacked this turn."
Witchstalker Frenzy 85%
$0.32#14728
"This spell costs {1} less to cast for each creature that attacked this turn."
Rowdy Research 85%
$0.22#9494
"This spell costs {1} less to cast for each creature that attacked this turn."
Embercleave 85%
$9.62#1324
"This spell costs {1} less to cast for each attacking creature you control."
+1 more matching line
Rally the Troops 81%
$11.18#26559
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Warrior's Stand 81%
$10.88#30153
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Scorching Winds 81%
$0.24#28959
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Deep Wood 81%
$0.47#24372
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Heavy Fog 81%
#24306
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Eightfold Maze 81%
#30188
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Champion's Victory 81%
$8.70#29102
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Defiant Stand 81%
$0.10#31337
"Cast this spell only during the declare attackers step and only if you've been attacked this step."
Harsh Justice 81%
#22811
"Cast this spell only during the declare attackers step and only if you've been attacked this step."