Riot is a replacement effect. Players can't respond to your choice of a +1/+1 counter or haste, and they can't take actions while the creature is on the battlefield without one or the other.
2021-06-18
wotc
If you choose for the creature to gain haste, it gains haste indefinitely. It won't lose it as the turn ends or as another player gains control of it.
2021-06-18
wotc
If this creature gets enough -1/-1 counters put on it to cause it to go to the graveyard, modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.
2021-06-18
wotc
If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste. In the case of Arcbound Slasher, however, it's toughness will probably be 0, and it will be put into its owner's graveyard before it can attack.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Legal
Timeless
Legal
Gladiator
Legal
Pioneer
Not Legal
Modern
Legal
Legacy
Legal
Pauper
Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Legal
Alchemy
Not Legal
Paupercommander
Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Not Legal
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