Yawgmoth, Thran Physician

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.

Wotc — February 04, 2023

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.

Wotc — February 04, 2023

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.

Wotc — February 04, 2023

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.

Wotc — February 04, 2023

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

Wotc — February 04, 2023

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.

Wotc — February 04, 2023

If a permanent has +1/+1 counters and -1/-1 counters on it, they're removed in pairs as a state-based action until that permanent has only one of those kinds of counters on it.

Wotc — December 08, 2022

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

Wotc — December 08, 2022

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.

Wotc — December 08, 2022

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.

Wotc — December 08, 2022

Protection from Humans refers only to the creature type Human. As far as Yawgmoth is concerned, you, your opponents, and planeswalkers aren't Humans.

Wotc — March 19, 2021

You may activate Yawgmoth's first activated ability without choosing a target creature. You'll just draw a card. However, if you choose a target and it becomes illegal before the ability tries to resolve, the ability won't resolve and you won't draw a card.

Wotc — March 19, 2021