Here is the official statement by blizzard.

In case you're wondering what the arena tournament is here's some information to help you out.
First of all it's a 3v3 arena tournament that takes place outside of the regular arena tournaments on the normal realms. You pay $20 and get access to a special arena realm where you can make up to 3 level 80s (you cannot copy players like ptr). On this realm all pve content is disabled and instead of spawning the normal 8 starter places you being in one of the special tournament spawn places. At the starter place players can get gold, enchants, gear, and gems. They then have a stage where they practice and once that stage is over the real tournament begins. Players who place in the top 1,000 overall receive The Vanquisher title. Any players who played at least 200 games will receive a special non-combat pet, Murkimus' Little Spear - Item - World of Warcraft. The top teams play a special ladder featuring a prize pool of over $200,000.
Last years winners was SK Gaming, there website can be found here: SK Gaming - Esports, Professional Gaming, Counter-Strike, Warcraft III, World of Warcraft, FIFA, DotA, Starcraft, Quake, Console

Who will win this year? Well only time can tell!
Good luck to all in the tournament and don't forget to go and register soon if you want in!
Sources: WorldofWarcraft.com