The Shimmermist Runner can be found in Mistveil Tangle, in the middle of the Tirna Scithe region in Ardenweald.
To obtain this mount, you need to go through the labyrinth at a certain order and kill Shizgher.
If you don’t follow the right path, Shizgher and the mount will despawn if you get close.
The correct path to Shizgher and the Shimmermist Runner involves traversing gateways with blue lamps on them.To start, you must find Tale of the Tangle at the Oaken Assembly and read it.
Then run directly across the area and pass through an arch made of branches, with a blue lantern hanging from it.
Enter the Maze in Oaken Assembly on (31.04, 54.51).
On the 1st intersection at (29.60, 55.76) go LEFT/SOUTH-EAST.
On the 2nd intersection at (30.02, 57.49) go RIGHT/SOUTH-WEST.
On the 3rd intersection at (29.56, 57.91) go LEFT/SOUTH.
On the 4th intersection at (29.31, 59.10) go RIGHT/NORTH-WEST.
On the 5th intersection at (28.82, 58.71) go FORWARD/NORTH-WEST.
On the 6th and last intersection at (27.56, 57.85) go LEFT/WEST into the Area.
You can now engage Shizgher.
Video
https://www.youtube.com/watch?v=qTKLGgkWfDc&t=9s&ab_channel=SoYouPlay
None the Wiser Guild 