Westmarch's history is one -- as is expected in the Diablo universe -- steeped in blood and conquest. The kingdom was founded by the Zakarum zealot Rakkis after a crusade that drove him across the Twin Seas and over the bodies of legions of conquered foes, but there is a second, "secret" history to this place. Near Westmarch is a "sprawl of fetid marshes," littered with the ruins of an ancient Nephalem civilization said to hold the power to ward against angels and demons alike. It was to these ruins that the now-mortal Tyrael took the Black Soulstone, and it was there that the former archangel Malthael, at that point neither angel nor demon, stole the Black Soulstone from its hiding place. And that, of course, is where the tale of Reaper of Souls begins. If you're a lore junkie looking for all the details, check out the full post on Diablo III's official site.