Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever this creature attacks, you may return target creature to its owner's hand if its power is less than this creature's power.
(24/11/2014)
Prowess triggers only once for any spell, even if that spell has multiple types.
(24/11/2014)
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.
(24/11/2014)
Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
(24/11/2014)
Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
