Sauron's entire high command, all going after a single warchief at once. He never stands a chance.
Confused about what happens in the clip? Shadow of Mordor grants players the ability control and dominate orcs, which in turn lets a player plant orcs in Sauron's high command. In this case, that power is used so that every single high-level orc turns on a single warchief. Given that these orcs are supposed to be the warchief's bodyguards, it's a betrayal of absurd proportions—and it was staged entirely by GnudGaming. And then to add salt to the wound, he kills off every single high-level orc in one go too, just like just like this person did.
If you don't have time to see the lead-up to the moment of betrayal, skip to the 2:15 mark—that's when you see the entire nemesis board turning on a single orc. Otherwise, enjoy the sounds of sped-up orcs. It's kind of funny.