Legendary chest appearance
First, the loot chests are on a very large rotation, and winning matches with full chest slots will not advance this rotation, so doing matches with full chest slots will not cause you to lose a rare chest. And you can get a legendary chest in any arena,with the legendary card being completely random.
Second, legendary chest appear in the shop for 500 gems after arena 7. Also, the arena 10 pack includes a legendary chest, and is $9.99. With a slight chance of one appearing randomly for $9.99 anytime.