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 , where X is the number of counters on that creature.
Nils is a mono-white pillowfort commander who hands out +1/+1 counters to creatures across the table each end step, then taxes those very creatures for attacking you. You build a counters-matters board while making yourself the worst attack target, surviving to grind out value and close with a few big payoffs.
Strong deterrent that scales—every counter you give makes attacking you cost more, protecting your life total and planeswalkers
Counters synergize with proliferate and counter-payoff strategies for explosive boards
Cheap 3-mana commander that immediately affects all players, including opponents' creatures
Mono-white consistency and access to powerful tax/stax and protection effects
Doesn't stop noncombat damage, mill, or attacks against other players, so it can be passive
Tax is per-creature and only protects you—wide token swarms can still find through-routes
Mono-white struggles with raw card advantage and ramp
Giving opponents counters can accidentally grow their threats or feed their own counter synergies
Legendary Creature — Human Cleric
, where X is the number of counters on that creature.