After you pass the last Dan, you will get into Leagues.
Leagues are a late-game activity which grants you valuable rewards at the end of every season. Each season lasts for a few days, and your reward is based on your League position at the end of this period. After that, your rating will be recalculated at the beginning of next season (this rule doesn't apply to Dans, though).
If you haven't reached Leagues yet - don't worry! We're happy to introduce season rewards for Dans as well. The reward is given out only one, for the highest rank received during the season. Booster packs contain a certain amount of Gems and a few high-quality cards.
Please take note that you will get your rewards only if you've participated in duel fights at least once during a season!