Cha'garoth's R'lyehian Gateway Key is the third Gateway Key, which requires the player to defeat Cha'garoth, The Dreadbeast, and be in possession of a Transmutator in order to craft. It can only be used within The Dreadlands and Omothol. Don't mistake the key for bringing you to R'lyeh, that's where it was originally forged.
Cha'garoth's R'lyehian Gateway Key is created by transmuting a Dread-plagued Gateway Key, which is dropped by Cha'garoth, The Dreadbeast.
If you right-click on the ground while holding this tool, it will form a portal. This will however not work if the path is to narrow, in which case nothing will happen (which would indicate that it was used at a incorrect location). If you happen to use it in a dimension that isn't the Overworld, the Abyssal Wasteland or the Dreadlands, it will also prompt a message pointing out that it isn't working.
- "R'lyehian" is a reference so the mythological city in H.P Lovecraft's books, where Cthulhu sleeps. The final dimension was meant to have some larger connection to the Cthulhu mythos, which it likely will. The reference was mainly pointing out that the key was forged in R'lyeh.