Even after his defeat at the hands of players in Diablo 3, the demon Diablo remains, as ever, a threat in Diablo 3: Reaper of Souls.
His soul is absorbed into the Black Soulstone, a gem which itself has great power thanks, and which becomes all the more dangerous thanks to its demonic inhabitant. The stone, and Diablo's soul, are banished from Heaven and are buried deep beneath the ground... and that is where our new story begins.
