This is the second expansion of Eldritch Horror and is a 'big box' expansion.  It adds a bunch of new cards, new investigators, new tokens, and a new side board.  If you're an Eldritch Horror fan this is worth the forty bucks as it not only brings new content but also adds more cards to the existing decks.  That said, however, I'd argue that getting the first expansion 'Forsaken Lore' before getting this is a better investment as Forsaken Lore adds even more cards to the existing decks which really adds to re-playability.  Mountains of Madness is still a great expansion, though, just not the first one I'd buy (but I did buy it).