The Stormcast Eternals are the primary image you get when you think of the Age of Sigmar. Since its release they have had a several waves of new miniatures and with the new Battletome coming up for pre-orders this weekend, the Warhammer Community has given us a preview of what to expect for the new book.
via the Warhammer Community
https://www.warhammer-community.com/2018/06/28/28th-june-battletome-preview-stormhost-rulesgw-homepage-post-4/
You choose the Stormhost for your army when you’re writing your list, giving each unit in your army a unique keyword representing which Stormhost they’re from. If a model already has a Stormhost keyword – like Vandus Hammerhand, for example – you can still include it in your army.