What is happening?(the bold words links)
This is a story of time travel and altering events. It's apparently a separate timeline so it won't mess with anything. Grom Hellscream (the main guy who refuses the cup) was supposed to drink the blood of Mannoroth (the big firey dude). This makes them super powerful, but also slaves to the Burning Legion.
Enter Garrosh Hellscream (the guy with the torch and tattooed chin guy), responsible for a civil war in the current Horde and only dreams of the old ways (blood and glory). He escapes a trial put on by the Pandaren for the destruction of the island and other horrific things he did there, and instead goes back in time to warn his father (the whole blood drinking thing) and helps create the Iron Horde who's main purpose is to conquer everything.
What is also interesting is Garrosh brings back technology. The giant spinning ball is something he used to fuel his war machines and it is neat to see it so crudely. So not only did he warn his Father, but he went back in time with a huge technology advancement.
Additional InformationBlizzard also wants people to know more about the enemies in the upcoming game and have created shorts to explain who they are. Here is a link to the trailer and also the first episode.
The amount of lore in the Warcraft Universe is crazy and if you read above, I hope you enjoyed it.