https://www.gamespot.com/articles/free-amazon-twitch-prime-loot-for-may-now-availabl/1100-6458638/

Each month, Amazon and Twitch give away free digital items to Amazon Prime members with Twitch accounts. This month, not only can you download six free games between now and May 31, but you can also get some free Heroes of the Storm loot as well.

Up for grabs in May are a Glitch-steed mount and five Rare Loot Chests. The contents of the chests are random, but each one is guaranteed to contain at least one rare item. Loot Chests in Heroes of the Storm contain a randomized assortment of things like heroes, sprays, skins, voice lines, banners, and announcers.