Mardu Warshrieker
Creature — Orc Shaman
Raid — When this creature enters, if you attacked this turn, add 
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"No body can contain so much fury. It reminds me of another battle, long past." —Sarkhan Vol
Illustrated by
Khans of Tarkir#117CommonEnglishLegality
Rulings
Mardu Warshrieker's raid ability isn't a mana ability even though it adds mana. It uses the stack and can be responded to.
Raid abilities evaluate the entire turn to see if you attacked with a creature. That creature doesn't have to still be on the battlefield. Similarly, the player, planeswalker, or battle it attacked doesn't have to still be in the game or on the battlefield.
Raid abilities care only that you attacked with a creature. It doesn't matter how many creatures you attacked with or which player, planeswalker, or battle those creatures attacked.