The Vertigo Series is a set of 20 new levels designed by the Gods themselves! The level design is awfully cool and you now got those cool lightning effects known from Doom. It also features 6 new robots and a continued storyline so it's definitely worth checking out. It was sold seperately and bundled with the Descent Mission Builder 2 - the best D2 level editor on this planet - as "Descent - The Infinite Abyss". Meanwhile you can get it only bundled like in "Descent - The Definitive Collection" at very reasonable prices.
Though the level design is quite twisted the Vertigo Series does not contain so many difficult scenes (especially not the secret levels, I didnīt even make a section about them!). Apart from that those levels really rock; those lightining effects are incredible.