Warren Instigator

Double strike
Whenever this creature deals damage to an opponent, you may put a Goblin creature card from your hand onto the battlefield.

Secret Lair Drop

Duel Decks: Merfolk vs. Goblins

Double strike
Whenever Warren Instigator deals damage to an opponent, you may put a Goblin creature card from your hand onto the battlefield.

Treasure Chest

"Danger! Danger! Come out of the safety of your holes!"

Zendikar

If Warren Instigator attacks an opponent and isn’t blocked, its double strike ability will cause it to deal combat damage to that opponent twice, once during each combat damage step. Its triggered ability will thus trigger twice: Warren Instigator deals first-strike combat damage, its ability triggers and resolves, it deals regular combat damage, and its ability triggers and resolves again.

Warren Instigator’s ability triggers when it deals any kind of damage to an opponent, not just combat damage.