Go through the sewers entrance and left of the ramp is where you need to be.
Face the wall with the 2 drains that overhang a pit of water.
Use Underbelly Elixir - Item - World of Warcraft until you become a bug.
Fly upwards pushing forward and flying towards the invisible wall. At the very top of the invisible wall when you are nearly touching the ceiling you should go forward and pass through the invisible wall.
Fly down and under the platform and you can fly under dalaran. You can manage to make your way to the dalaran arena before getting dismounted.
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