https://www.gamespot.com/articles/blizzard-denies-diablo-reign-of-terror-is-blizzcon/1100-6462515/

We may see some form of Diablo announcement at BlizzCon this year, but a set of merchandise with a new name isn't an accidental leak, according to the studio. A set of new items listed on the Blizzard Gear Store raised eyebrows with a conspicuous subtitle: "Diablo Reign of Terror." However, Blizzard has told GameSpot these are not related to any new content being prepared for the show.

The items include a poster and a men's and women's T-shirt, all showing the frightful visage of the series namesake himself. That raised suspicions that "Reign of Terror" could be the subtitle of a new game or expansion, especially given that Diablo hasn't had either since Reaper of Souls in 2014. But not so, says Blizzard.