Typheus

The Denizen of the Land of Wind and Shade. According to the salamanders, he is kind of a huge dick (probably not literally).

The Slumbering One is a person who 'put a spell' on the Fireflies trapping them under the clouds.

He was asked, in the past, to commission a bunch of Underlings. After this The Slumbering One filled the Land of Wind and Shade with the black liquid which blocked some of the Parcel Pyxis.

Sleeping sounds were heard emanating from a large pipe in the area John first discovered. It is possible that The Slumbering One is currently deep underground.

It is hinted that John is supposed to slay this person to free the salamanders and their land... and fireflies... near the end of the game. However, the author-stated layout of the gates suggests that Rose should have been the one to fight the Land of Wind and Shade's Denizen instead. It remains to be seen if this is a contradiction or not, as none of the Kids are exactly adhering to the assigned structure of the game at this point.

In an alternate timeline, John is killed because he fought The Slumbering One too soon, thanks to the advice of Terezi, and thus could not bring Jade into The Medium, most likely resulting in her death as well.

The term "Slumbering One" may refer to Typheus, the name of John's browser, as well as the probable identity of the Denizen. This is supported by the hints that the Denizen is deep underground. After being defeated by the Gods, Typheus was trapped underground