At the beginning of your end step, if a legendary creature you controlled attacked alone this turn, collect your reward.

Before the game, shuffle at least 6 unique bounty cards into a face-down pile.
As the starting player's third turn begins, reveal the top bounty card.
Claim the revealed bounty during your turn and collect your reward!
As each turn begins, if no bounty is being offered, reveal the next one. If the pile is empty, shuffle all claimed bounties and restock
If the bounty went unclaimed last turn, increase its reward to the next level.
Rewards
1 — Create a Treasure token
2 — Create two Treasure tokens
3 — Create two Treasure tokens *or* draw a card
4 — (Max) Create two Treasure tokens *and* draw a card.