Halo
Halo 5 and Halo Wars 2 Information
Hearthstone Cards Information.
Hearthstone API is a free Games & Comics API that requires X-Mashape-Key authentication, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Hearthstone API is listed in the Games & Comics category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Hearthstone API uses X-Mashape-Key authentication, so you need to register for credentials before making requests.
CORS support for the Hearthstone API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Hearthstone API is served over HTTPS, so it's safe to use in production.