Nils, Discipline Enforcer {2}{W}
Legendary Creature — Human Cleric
At the beginning of your end step, for each player, put a +1/+1 counter on up to one target creature that player controls.
Each creature with one or more counters on it can't attack you or planeswalkers you control unless its controller pays {X}, where X is the number of counters on that creature.
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Cards that do something similar expected a card that isn't here?
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Lovestruck Beast // Heart's Desire 84%
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"This creature can't attack unless you control a 1/1 creature."
Demon Wall 84%
$0.23#16021
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Mogg Toady 83%
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Goblin Goon 83%
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Champions of Minas Tirith 83%
$0.32#5273
"At the beginning of combat on each opponent's turn, if you're the monarch, that opponent may pay {X}, where X is the number of cards in their hand. If they don't, they can't attack you this combat."
Backstreet Bruiser 82%
$0.09#26117
"As long as there are two or more counters among creatures you control, this creature can attack as though it didn't have defender."
Warden of the Chained 82%
$0.10#27889
"This creature can't attack unless you control another creature with power 4 or greater."
Unstable Glyphbridge // Sandswirl Wanderglyph 82%
$0.29#8137
"Whenever an opponent casts a spell during their turn, they can't attack you or planeswalkers you control this turn."
Promise of Loyalty 81%
#970
"Each player puts a vow counter on a creature they control and sacrifices the rest. Each of those creatures can't attack you or planeswalkers you control for as long as it has a vow counter on it."
Collective Restraint 81%
$14.61#6738
"Creatures can't attack you unless their controller pays {X} for each creature they control that's attacking you, where X is the number of basic land types among lands you control."
Lambholt Pacifist // Lambholt Butcher 81%
$0.21#23485
"This creature can't attack unless you control a creature with power 4 or greater."
Akron Legionnaire 81%
#28174
"Except for creatures named this card and artifact creatures, creatures you control can't attack."
Arcades, the Strategist 81%
$4.32#5803
"Each creature you control with defender assigns combat damage equal to its toughness rather than its power and can attack as though it didn't have defender."
Lurking Green Dragon 81%
$0.21#22222
"This creature can't attack unless defending player controls a creature with flying."
Total War 81%
$2.49#19506
"Whenever a player attacks with one or more creatures, destroy all untapped non-Wall creatures that player controls that didn't attack, except for creatures the player hasn't controlled continuously since the beginning of the turn."
Web of Inertia 80%
$0.75#11836
"At the beginning of combat on each opponent's turn, that player may exile a card from their graveyard. If the player doesn't, creatures they control can't attack you this turn."
Archangel of Tithes 80%
$4.74#2212
"As long as this creature is untapped, creatures can't attack you or planeswalkers you control unless their controller pays {1} for each of those creatures."
Orcish Conscripts 80%
#30294
"This creature can't attack unless at least two other creatures attack."