Core Prowler — ruling 1

To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.

Core Prowler — ruling 2

You don't have to choose every permanent or player that has a counter, only the ones you want to add another counter to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.

Core Prowler — ruling 3

If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.

Core Prowler — ruling 4

An ability that triggers "Whenever you proliferate" triggers even if you chose no permanents or players while doing so.

Core Prowler — ruling 5

Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.

Core Prowler — ruling 6

If a permanent ever has both +1/+1 counters and -1/-1 counters on it at the same time, they're removed in pairs as a state-based action so that the permanent has only one of those kinds of counters on it.

Core Prowler : règles, fonctionnement et interactions (MTG)

Core Prowler is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.) When this creature dies, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Complete guide

Core Prowler is a relentless Phyrexian Horror that ensures its influence lingers long after it leaves the battlefield. By combining the deadly Infect mechanic with a powerful death trigger, it serves as both a threat and a catalyst for your counter-based strategies.

Spreading the Contagion

Core Prowler features Infect, meaning it deals damage to players as Poison Counter(s) and to creatures as -1/-1 counters. Unlike standard damage, this permanently weakens blockers or moves a player closer to losing the game. When Core Prowler dies, its triggered ability allows you to proliferate, letting you add an additional counter of every kind already present on any number of players or permanents you choose.

Resolving the Proliferate Trigger

When Core Prowler is put into the graveyard from the battlefield, its ability goes on the stack. Upon resolution, you first choose any number of players and permanents that already have at least one counter. For each chosen target, you add one more of every counter type they already have. Note that if a permanent has both a +1/+1 counter and a charge counter, it must receive one of each; you cannot choose just one kind. However, you can choose to proliferate zero targets if you wish.

Common Mistakes

A common misconception is that you can choose which specific counters to add; in reality, you must add one of every type already present on the chosen target. Another error involves timing: players can respond to the death trigger while it is on the stack, but once you begin choosing targets during the resolution of proliferate, it is too late for opponents to take actions like removing a permanent to prevent it from getting a counter.

Strategic Deployment

Core Prowler is most effective when you already have established counters on the board. Since it has Infect, it can create the very Poison Counter needed on an opponent to later proliferate that player. It is an excellent deterrent against attackers, as your opponent must weigh the downside of losing their creature to -1/-1 counters against the risk of triggering your proliferate effect upon the Prowler's death.

Interaction examples

Doubling the Prowl

Roaming Throne

If you control Roaming Throne with 'Horror' chosen as the creature type and Core Prowler dies, Roaming Throne causes the death ability to trigger an additional time. You will resolve the proliferate effect twice, potentially adding two Poison Counter(s) to an opponent who already had at least one.

Value in Death

Solemn Simulacrum

If both Core Prowler and Solemn Simulacrum are destroyed by a board wipe, you can order the triggers so you draw a card from Solemn Simulacrum first, then resolve the proliferate trigger from Core Prowler to increase counters on any surviving permanents or players.

Infectious Combat

Poison Counter

If Core Prowler attacks an opponent and is not blocked, it deals 2 damage in the form of two Poison Counter(s). If that player then uses a spell to destroy Core Prowler, its death trigger allows you to proliferate, adding a third Poison Counter to that player.

Guide written with AI from the official text and rulings. Official English text prevails.

How it works : Core Prowler

Original English text — Wizards of the Coast / Scryfall

#1February 4, 2023· Wizards of the Coast
To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.
No other card is named in this ruling.
#2February 4, 2023· Wizards of the Coast
You don't have to choose every permanent or player that has a counter, only the ones you want to add another counter to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.
No other card is named in this ruling.
#3February 4, 2023· Wizards of the Coast
If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.
No other card is named in this ruling.
#4February 4, 2023· Wizards of the Coast
An ability that triggers "Whenever you proliferate" triggers even if you chose no permanents or players while doing so.
No other card is named in this ruling.
#5February 4, 2023· Wizards of the Coast
Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.
No other card is named in this ruling.
#6February 4, 2023· Wizards of the Coast
If a permanent ever has both +1/+1 counters and -1/-1 counters on it at the same time, they're removed in pairs as a state-based action so that the permanent has only one of those kinds of counters on it.
No other card is named in this ruling.