Not the best expansion pack Diablo could get.
Now such an influential game definitely deserves a substantial expansion pack to live up to its name. Hellfire fails at this. It's way too much on the safe side and only does the basic thing expansion packs do. Adding content. It never tries to make a big improvement or change. It doesn't add anything that's gonna be missed by the people who don't experience them. it's just more of the same.
Hellfire's story is very insignificant and doesn't add anything of value to the universe. No wonder it's completely ignored in Diablo 2.
It's basically about an unleashed demon named Na-Krul who was unknowingly summoned by a mage during a ritual, but the mage sealed the door to the room which Na-Krul was in before he could completely escape. Now our hero must venture in 8 new levels and reach the demon to tell him he's not welcome. There are some journals and a hidden scroll left by the mage which express his fear and trauma caused by the incident, but other than that, you can get the whole story from the short intro made for HF.
8 new levels in the game have two different style. 4 of them are festering nests which its enemies and environment kinda reminded me of Starcraft Zergs and has a small sub-story about an insect-like monster called the defiler who wants to conquer humans or something like that, but obviously fails because it has to face the player and other 4 levels are demon crypts which Na-Krul inhabits at its 4th level.
You can't access Hellfire levels at first and you have to get past a certain point in the main game. This gives the opportunity to see the little effects Hellfire has on the main game. There are new shrines, spells, items, etc and also new stuff like runes which act as a trap and oils which restore your equipment's durability. But all these new additions are not very useful and make little difference.
Although the new class, monk, is a good addition. He's basically something between all of the three main classes and he's also good with staves. Monk is very strong if you develop and equip him correctly and defeating Diablo ( and the game in general ) with monk was a lot easier compared to my play-through with the sorcerer.
News areas in the game ( nest and crypt ) have a good design and are a nice change from the main 16 levels. Nest delivers the feeling of a gooey and slimy insect ridden place and Crypt has a suitable Gothic feeling to it. Also the same thing can be said about the new enemies and although they basically have the same attack techniques and they're like the old enemies with a new appearance, their looks suit the environment they're in. Although their dying animation is not as satisfying and detailed as Diablo monsters.
Overall, Hellfire is not bad. But that's mostly because it doesn't take any risks. It just adds a bunch of stuff which can bring a couple of hours of more single-player fun for the fans ( HF doesn't support MP which wan't a problem for me, but it must have been terrible back in the day it was released when many people played Diablo for its multi-player ), but Diablo definitely deserved a more ambitious and groundbreaking expansion pack.