Priority Avenger

Flying
Players can't cast instant spells unless a spell or ability is on the stack.

Mystery Booster Playtest Cards 2021

Mystery Booster Playtest Cards 2019

Among abilities, only activated and triggered abilities use the stack. Activated abilities are written in the form “[Cost]: [Effect].” Triggered abilities use the word “when,” “whenever,” or “at.” They’re often written as “[Trigger condition], [effect].” Some keyword abilities are activated abilities and will have colons in their reminder texts, and some keyword abilities are triggered abilities and will have “when,” “whenever,” or “at” in their reminder texts.

Non-instant spells with flash may still be cast any time a player has priority, regardless of what’s on the stack. The same is true of activating abilities.