The ruins of lordaeron is often simply called the lordaeron Arena.
It is located in Tirisfal Glade and can be entered in the game world, however unlike the 2 other arenas, it is not a Free For All pvp zone, but rather the entrance to the Horde Capital - the Undercity. As such going there as an Alliance player is considered rather unhealthy.
It is also one of the 5 current possible location for Arena battle.
The ruins of lordaeron at first appear to have a lot going on, but the ghosts and green slime are only there for ambiance and have no bearing whatsoever on the fights. The two key difference between this arena and the other are the tomb at the middle, which can be used to break line of sight as well as the small nook in the entry zone, which allow players to remain hidden and prevent line of sight with fighters in the arena. Most strategic element about this arena involve using the tomb to break line of sight, or using the nooks in the entry zone.
Like all Arenas, after 5 minutes, 2 Shadow Sight power ups will appear on each side of the arena. When a player touch the power up, he gets a debuff that greatly increase his stealth detection but also increase all damage taken by 5% for several seconds.