Corridor Creeper mount is Very special，This mount is a reward for completing 8 Layers of Twisting Corridors. 
The 'challenge mode' of Torghast. apparently each layer takes 1 to 2 hours to complete. Throughout 9.0 Shadowlands, players need to spend a lot of time challenging torghast.
There are 7 areas in total, and two areas are refreshed every week
Each area has 8 Layers and each level has 6 levels
progress doing the each layers Reward :
Layer 2: Reward:  Death Seeker.
Layer 4: Reward:  Helm of the Dominated.
Layer 6: Reward: Spirestalker %s Title.
Layer 8: Reward  Corridor Creeper.
It requires 195+ ilvl to complete this achievement.
If needed, you can purchase WOW Gold & BOE Gears & Layer 8 Torghast, Tower of the Damned Clear to upgade your ilvl!