Blizzard North never released an expansion for Diablo.
The only sanctioned expansion is Hellfire, made by a different company, (I forget who).
Blizzard North did borrow, (argue ably) many features from Hellfire that made it into Diablo 2.
The monk was badass! I remember being stuck on where to go next at one point because I couldn't find who gave the next quest I needed. Took me forever to figure out that I needed to talk to Gillian. She is so useless that I had honestly forgotten that she was in the game!
It's fairly reasonable for a cheap little 3-rd party expansion. Interesting enough D2 has some elements found exclusively in Hellfire, such as the ability to highlight/see which items are on the ground without mousing over them. Also, Hellfire enables Jogging. The quests aren't terrible and the classes are fun if overpowered.