The latest update for Clash Royale added a new purchasable bundle that is tied to your arena rating. It will pop up each time to reach a higher arena, and offers a combination of gems, gold and a legendary chest for the highest arena. It costs 10 euros, and you can buy one for each arena rank.
The higher the arena you are in, the better the pack is. Arena 9 is the best, with 1 200 gems and 100 000 gold + the Legendary Chest.
When you hit a new arena, the pack will pop up for sale for two days, and after that it will only show up for sale once per week. Once you buy it, you cannot buy a new one, but you will have a 25 % chance to be offered an arena pack from a lower arena. So if you are arena 9, you can still buy arena 8, 7, 6 etc once you have bought the arena 9 pack. It's possible to buy all the arena packs, but it will take a little time to get the option to purchase each one because of the 25 % chance.
Once you buy an arena pack (e.g. arena pack 9), you will never be able to buy it again. The pack is obviously of very high value, so any serious players should buy this before buying other stuff in the game.
The Legendary chest from the pack opens immediately, so there is no need to wait. You also get both gems and gold immediately once you buy the pack.