Prowess

Prowess is a keyword ability that was introduced as the clan-specific mechanic for the Jeskai Way in Khans of Tarkir and Fate Reforged. Creatures with prowess get +1/+1 until end of turn whenever a noncreature spell is cast by their controller.

Description
Prowess was designed by Jonathon Loucks during the second Great Designer Search. R&D's need for blue to get a combat-relevant keyword that wasn't about evasion caused prowess to become evergreen in Magic Origins but was phased out as such in Hour of Devastation since it had too many issues. It is still deciduous.

Prowess is secondary in red and tertiary in white. Mark Rosewater has admitted that prowess has more design issues than the average evergreen mechanic. Problems typically included the density of cast triggers (overlapping with historic triggers in Dominaria ), weakness in creature-dense sets (such as for the Merfolk tribal theme in Ixalan ), and coverage of artifacts and enchantments being unfavorable (such as for the Izzet League in Guilds of Ravnica wanting only instants and sorceries), leading to the keyword constantly being pulled off sets. Because of this, prowess was in danger of losing its evergreen status and was ultimately phased out.

Prowess wasn't banished forever, it was just being phased out of being evergreen. It was already used again in Commander 2019 and Core Set 2021.

Rulings

 * Any spell you cast that doesn't have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won't cause prowess to trigger. Playing a land also won't cause prowess to trigger.
 * Prowess triggers only once for any spell, even if that spell has multiple types.
 * Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
 * Once it triggers, prowess isn't connected to the spell that caused it to trigger. If that spell is countered, the ability's effect will still resolve.