Corridor Creeper is the reward from completing Twisting Corridors: Layer 8.
Twisting Corridors is a special, longer version of Torghast, which has some cosmetic rewards attached to it.
Since Twisting Corridors was released in Patch 9.0, it is fairly easy using gear from Patch 9.1 and beyond.
For clearing Layer 8, you will earn the Corridor Creeper mount.
This mount was originally very notable, as it is one of the only mounts that can be used in The Maw, before Korthia made all mounts work in the Maw.
Unlocking the Twisting Corridors
Twisting Corridors is not automatically unlocked together with Torghast.
To unlock Twisting Corridors, you will need to complete the questline given by Highlord Bolvar Fordragon.
This questline starts once you complete the Covenant Campaign quest The Search for Baine.
Each week, Bolvar will give you one quest that requires you to go back to Torghast and search for specific things, or run a special wing to rescue one of the imprisoned Azeroth leaders.
NOTE: The Twisting Corridors unlock is account-wide, which means completing the questline and unlocking Twisting Corridors once will unlock it for all characters in your account.
NOTE: If you have Korthia unlocked, you have these quests completed already.
Explore Torghast
Remnants of Hope
Torment Chamber: Jaina + Information for a Price
Finding a Witness
Lest the Trail Go Cold
Torment Chamber: Thrall
Signs of the Lion
None the Wiser Guild 