Level 3 of the Gladiator's Sanctum unlocks the Highmaul Coliseum, shown in the video above. A solo scenario, the Coliseum places you in a pit with other players. Once the event starts, it's a free-for-all brawl, and the last person standing wins the spoils. What kind of spoils? PvP gear, of course! To unlock the level 3 blueprint for the Gladiator's Sanctum, players must first complete The Bone Collector, an achievement that requires players to collect 4,000 Broken Bones from PvP combat in Draenor -- any kill that awards honor also awards 3-6 bones. Once you've built the level 3 Sanctum, you gain access to Highmaul Coliseum, as well as a buff that reduces all damage in outdoor Draenor zones by 50% if you fall below 35% health.
Take a look at the video for more of the Coliseum in action -- I particularly like how dead players are teleported up into the stands to watch the rest of the brawl play out, spectator style.