"Your journey begins on the trail of a group of mysterious cultists causing trouble in Sanctuary. As you hunt them down in Adventure Mode, you’ll uncover clues leading to a portal into Tristram’s past, and the terrifying darkness that took hold of the town so many years ago. . . .
Follow the clues and you’ll soon find yourself in a realm of glorious RetroVision™, where an all-too-familiar cathedral looms. Explore the depths and you’ll discover familiar enemies and iconic items, all brought to life in the Diablo III engine. Meanwhile, deep beneath the catacombs, the Dark Lord himself awaits any intrepid adventurer who dares to challenge him. . . .
What is a Diablo game without shiny new loot to call your own? Completing activities in the anniversary event yields unique transmogrification effects, achievements, portraits, pets, and more.
While some rewards are easy to find, others may have you scouring every nook or unlocking every achievement before you can get your hands on them. Can you earn them all? There’s only one way to find out!"
I missed this last year so I think I'm going to take a quick peek and see how it the event plays!