The player-versus-computer co-op mode stars Genji, McCree, a pre-Reaper Gabriel Reyes and Moira set on the game's new map Venice (which will arrive in the PTR before being added to the game's rotation). It'll play a lot like last year's Uprising event, which you can also dive into until the end of the month. Overwatch has rebranded both events under the Archives umbrella, cementing this season as a look back into the game lore's past, and added plenty of new skins (including the black ops-style outifts Moira and Reyes wear in the Retribution mode) to earn.
Just be ready for a very large patch...to the tune of 18GB on desktop and 22GB on console. Blizzard tweeted out a heads-up, and the game's team noted that the huge update includes data layout improvement to help loading times in the future.