The Gauntlet consists of 15 waves. After each wave is complete, a reward of a random item (shards and Heroes included) is added to a large number of coins. The Gauntlet can be reset once per day. Unlocks at LV 30.
After every successful wave completed, you are offered a brown chest. Brown chests contain gold and random items; the amount of gold and rarity of the items increase the further you go in the gauntlet. After every 3rd victory, you will receive a red chest. Red chests contain Dragoncoins, gold, and better items. It is not uncommon to receive blue or purple items, shards, or even tier 2 or 3 heroes from Red Chests.
There are a total of 12 items sold here: 2 are Prometheus and Hecate shards at 500 Dragoncoin for 5 shards, 2 XP items, and the other 8 are blue and purple items.