There’s a ton of imagery and Grimore to suggest the tetrahedron ships are the “Darkness”. The opening cinematics of the campaign show a bunch of pyramids falling into the water (the Deep), the Traveler being chased by a big black triangle etc.
My thought is that there are multiple Travelers (or there used to be and maybe ours is the only one left) and the Darkness’ mirror to the Travelers are these giant black tetrahedrons. So that they aren’t actual “ships” carrying a new species, but “Dark Travelers” coming to hunt ours down. I’m assuming they would bring with them the powers of the Darkness to bestow on species they think are worthy, like the Hive.
Their appearance is completely opposite of the Traveler, so that makes me think they are the “dark mirror”.
Traveler = white and round
Tetrahedron = black and pointy
My thought on that concept art showing the 4 races is that the last race with the triangles looks a lot like Taken enemies with the big white eye. Maybe the Dark Travelers bring a bunch of different races of Taken enemies with them.