HeXen: Beyond Heretic + Deathkings of the Dark Citadel Whereas Heretic was a game very similar to the original Doom, HeXen has taken some steps to ensure that it feels different, despite the fact that it's also using id Tech 1.
While it retains the same atmosphere/setting and the same item system, HeXen says goodbye to Heretic's episodic and linear level progression. Instead, it has been replaced with hubs, with each of them containing stages (think like Super Mario 64), which you end up revisiting more than once tlll you can find the way out of the hub and into the next one. Also unlike Heretic, HeXen offers the choice among three characters (fighter, cleric, mage), with each of them having different stats, along with different weapons. Thus, while the fighter is mostly a melee dude with high speed and armour, the mage is slow, weak but can take enemies from afar and the cleric is a nice balance between those two. Another neat thing is that some of the items' effects differ depending on the character. For example, while the Flechette acts like a throwing bomb at the hands of the fighter, it acts like a poison cloud at the hands of a cleric and as a timed explosive mine at the hands of the mage. Thus, replaying the game with another character will definitely feel different.
Having said that, I'm not totally impressed with HeXen's differences. While being able to choose among three differing characters is defiitely nice, I can't help but feel that the feature is underdeveloped. Being limited to only 4 weapons per character (as opposed to Heretic's 8 weapons), combats can end up feeling monotonous at times, a feeling that is exacerbated by the lack of tomes of power or even a secondary usage. Add to all this the possibility that you might not get the 4th weapon (it's split into pieces and you might end up missing some), or that even if you get it, it expends a lot of mana from both of your pools and you'll fall into the routine of using your 2 mana-dependent weapons over and over again and the small variety of enemies further reinforces this routine. On a bit of an unrelated note, I'm also not a fan of the way that level progression works, as the backtracking can be a bit too much at times.
In conclusion, HeXen: Beyond Heretic is a good game, with some interesting differences from Heretic that aren't, unfortunately, enough to make it as enjoyable as it (heck, it doesn't even have some memorable music. Boo!).
What about the Deathkings of the Dark Citadel, though? Let's see; no new music, no new level layouts, no new monsters and no new weapons/items. It's a mishmash of the levels from the base game with even more enemies, which ends up degenerating into a snorefest. Making matters worse is the changed respawning feature. In the base game, there might be the occasional 1-2 weak enemies respawning every 2 minutes or so, to prevent a revisit to one of the stages from feeling completely empty. in the expansion, 5 or more of the annoying enemies are respawned, which makes the already boring combat an even bigger chore. My personal recommendation is, thus, to only play the base game and to not bother with this waste of time.
Note: If you are going to play the game with a source port, I advise you against using Doomsday Engine, as saving and continuing the game might result in crucial doors not opening up and thus, preventing you from completing the game. I found that out the hard way and I ended up switching to GZDoom, where I faced absolutely no problem.
Updated the full list.