Action packed, but with a decent story.
Single, without henchmen.
Difficulty is set at very difficult.
Main Classes used: Rogue, Druid, Sorcerer, Ranger, Cleric, Fighter and Monk, all started at level 15
Gameplay:
I never bothered to include any henchmen since I've already pushed the slider to very difficult, and it would be moot to include any helping hand. It is reasonable to assume that at "normal" difficulty, this would be very easy.
There would be some contest to what I'm going to say but this really felt like a hack and slash game. Enserric the longsword was a powerful weapon to begin with. The only class I could remember having difficulty at the beginning was my noobish rogue character against the blue dragon. Aside from this, the first module was fairly balanced for all the classes in my experience.
However, the most striking difficulty I encountered was at the end of the second module, the melee classes that are not spell dependent have difficulty against hordes of low level enemies (yes, hordes is not just a name, you get to fight them). I had to prepare making sure I have great cleave, whirlwind attack plus other area of effect attacks. Spell casters however may find the first few moments too easy but during the length of the battle, there is no time to rest and if you don't plan your castings well, you'll end up exhausting all of your memorized spells.
The third module's difficulty is highlighted by the end game boss. Make no mistake, this is a very skewed encounter. Spell casters are bound to die if you don't have specific spells prepared like "Mestil's acid sheath". Some of my melee capable characters like a buffed up cleric, a fighter and a rogue/shadowdancer almost killed Mephistopheles in minutes (probably seconds excluding the elemental sideshows). This polarized encounter is one of my main gripes about this expansion.
However, I still use this game for a quick research on character builds.
Story
There are concepts reminiscent of Discworld (IMO) like the mimic and Enserric the longsword. The story is darker, and more intense. I found it to be engaging. The barren landscape echoes this ambience and does not detract with the overall experience. There are no grasses, no windswept trees, no beautiful woods (well, one did appear albeit an illusion) but this appearance was matched by the overall mood of the story. Aribeth ending up in Cania was a clear departure from the "happy ending" possibility in the OC. Aarin Gend mentioned in passing that Lord Nasher rarely makes the same mistake, plus the hinted love in the jail talk with Aribeth makes the player think that there could possibly be a happy ending for this tragic character. However, with this expansion, we can all put a rest to that and say she ended up badly especially if you bring her with you without much points in persuasion to face Mephistopheles.
Graphics
Obviously dated but I must admit, there are games that I can't play right now because of their dated graphics but this aurora engine is still playable to this date.
Music
What can I say? It definitely ranks among the best. The music in this expansion is up there with Divine Divinity, Beyond Divinity, and the Total War series. The percussion and chords in minor are definitely in tune with the overall mood.
Overall
Definitely a timeless game. Neverwinter Nights Diamond is installed on my hard drive for one reason only and that because of HOTU.