Which expansion added the 'Player versus Player' arena system?

The expansion that added the “Player versus Player” arena system was The Burning Crusade.

Released in January 2007, The Burning Crusade introduced Arenas as small-team, instanced PvP matches. Players could compete in 2-versus-2, 3-versus-3, or 5-versus-5 formats, creating a more structured alternative to battlegrounds and open-world combat. Rated matches used a ladder and awarded points that could be spent on powerful PvP equipment.

The expansion launched with three arena environments: the Ruins of Lordaeron, the Circle of Blood, and the Ring of Trials. Arena teams also became a major part of World of Warcraft’s competitive culture, eventually supporting official tournaments and esports events.

A common mix-up is confusing Arenas with battlegrounds. Battlegrounds existed in the original game, while The Burning Crusade added both the Eye of the Storm battleground and the separate Arena system. Players without the expansion could enter some skirmishes, but rated Arena competition and its rewards required The Burning Crusade.