In T5 or above maps, there is a chance that a wisp(sparkling orb) will spawn. Go near it and it will open a portal to "The Mists."
T5 maps are marked as Yellow Region, which has PVP, so you have a chance of getting killed by other players. The only consequence of this is losing item durability and being unable to do anything for 3 minutes. Be warned that getting killed by another player in the Mists will warp you back to your previous map.