Bro the earth would probably be tidal locked with the traveller, meaning no day night cycles. Unless the traveller reflected some of the suns rays back onto us. We wouldn't be able to live underneath it or anywhere on earth. The planet would be uninhabitable. You would have a better time growing potatoes in a white dwarf. Unless the traveller goes against special relativity and all of physics. It would literally have to break the universe to exist that close to the planet without ill effects. It would be more likely that the traveller exists both in and outside this dimension, and what we see is just it's "shadow". A being of such immense strength that it can project it's power between dimensions is scary as hell. The traveller isn't followed by the darkness, it causes it. By tearing through our dimension it's created a rift in which the void can seep through. The void being the space between dimensions, it's vast and endless. There is no way to escape or defeat it. Nowhere you can run or hide will save you. It will consume. But not to worry, there a an infinite number of alternate time lines where the traveler didn't choose our universe. It chose a different one in a different dimension.