Not sure if you guys play this already or not.
You split up into 2 teams, 8v8 or something like that. The goal is to defeat the other team's leader, which you have no idea who it is. Before the fight starts each team declares said leader among themselves and informs the herald of who it is and the fight begins. Its a pretty simple really, you keep killing the other team until you kill the leader, after doing so, the herald will declare the team leader has died, and the other team has won.
Its kind of nifty because often people select their best player as a leader, which is predictable and will often lead to that person getting singled out, so mixing up who you pick as a leader is important.