Diablo IV's announcement on BlizzCon 2019 is all but confirmed but new leaks and information keep appearing online. Recently, a new artbook was put on pre-order at Amazon Germany, that originally mentioned Diablo IV. That display is now gone, but a new page from the book was leaked by insider WeakAuras, a page dedicated to Lilith, mother of Demons.
This character first appeared in Diablo II, for a brief moment, but if this page is real, it seems to indicate Lilith will play a much bigger role in Diablo IV, complete with a re-design of the character (kinda reminds us of the Queen of Blades from StarCraft).
Again, we can't confirm this page is real or not, so take this info with a grain of salt. Regardless, BlizzCon 2019 is just around the corner, on November 1 and 2, and all shall be confirmed or denied.
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