The good news here is that this addition is utterly free. If you own Eldritch, simply download the DLC once it's available, and it's all yours. The bad news is that beyond that we know very little. Minor Key Games has released the above trailer for the DLC, but unless the most horrifying thing you can imagine is inexplicably huge penguins, the footage is scant on the kind of psychological horror players would hope for from a DLC named for one of the Lovecraft's most famous stories.
With less than two weeks left until Mountains of Madness debuts, it seems we'll have to wait until then to see exactly what it adds. Personally, I'm hoping for something squamous, but I could easily go for a few unknowable geometries or an antediluvian deity representing entropy itself.