A new teaser page appeared on the official Diablo 3 site, with the name Reaper of Souls in it and an image having the "d3x_teaser-en-us.jpg" filename. Although it's not confirmed officially, there's a good possibility that this could be the expansion.

The first Diablo III expansion is "Reaper of Souls", teaser site is live [NeoGAF]

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