Night Fiend is highly useful against swarm Lil, Golem Zirix and some other less prominent swarm archetypes. Also it can clear full board of poop dervishes, rag eggs and distant 2 health heartseekers. It covers more ground than paragon thus it is better for killing large swarms.
The flow of the deck is similar to old Disruption Faie. You allow the opponent to do nothing while slowly proceeding to your inevitable wincon. Removal is essential to the playstyle I enjoy with this deck. Given considerable amount of draw and cycle, you rarely run out of cards. And the fact that the removal spells are cheap allows you to destroy enemy minion and develop some board in form of, say, Ooz.
Speaking of the general flow of the deck, I believe I described some points in the write up above. If you have some particular questions just ask.
Tbh, however, I’m still getting used to the deck in a sense that sometimes I still make obvious mistakes during mulligan and replaces. That said I reached gold 6 from rank 22 pretty fast mostly using this deck and will test it in diamond soon.
@project2el, you are free to look at my matches if you wonder what I mean. They are not perfect, since I’m not the best pilot, but I can assure you the deck has potential to be S-worthy (not at the top, of course) and you can get the rough idea of how I view control Cass.