The two missing merchants with some extra special items inspired by Icewind Dale and Planescape: Torment. Once you download the zip, simply unpack both .bcs files to your Override directory and Voila!, the outlets to the extra items are there.

Deidre will sell a variety of powerful equipment near the back of the Adventurer's Mart in Waukeen's Promenade, while Joluv now sells a variety of powerful equipment in the Copper Coronet. (He stands next to the grill in the middle of the room).

WARNING: This is the original version of this mod. However, a WeiDU version with many bug fixes and compatibility with other mods is also available, so it is recommended that you install that one instead. You can find it here.

The Ascension isn't all about making the combat more challenging (like other unofficial stuff by David Gaider below). It's about making the ending to Throne of Bhaal longer, more fun and a little more fulfilling. The mod mostly affects the end part of the game, starting at the very end of Chapter 9 when you meet with Balthazar and climaxing at the site of the ascension in the Throne of Bhaal.
There are numerous changes if you play the whole expansion as well, but please read the included txt since if I post them here I would spoil those who haven't finished ToB too much.

NOTE: You MUST read the readme file included in the zip first, where the author explains exactly how the whole thing works and what you should look out for!

Bhaalpowers Addition allows the player to gain new abilities after completing each challenge in the Abyssal Pocket Plane that were originally planned; the abilities will appear under the "special ability" icon and which ones are received depends on the dialog with the Solar prior to each challenge.
As David Gaider explained: "We originally had planned out some Bhaalspawn powers that the player would accumulate after doing the challenges in the pocket plane. It was scrapped because it was thought that these extra abilities plus all the high-level abilities would get too confusing. All of the abilities are still in the game, though, so it was minor scripting to add them back in for those people who would like to try them."
As the abilities were originally by designer intention, and removed only because of possible confusion and not due to throwing off game balance, they can be called a "tweak." The most recent (minor) update adds the full descriptions of the abilities into the ReadMe, as several players were wondering what the abilities did.

Original Tougher Demogorgon v2.1 restores the Prince Of Demons to his originally planned difficulty, before he was "toned down" for game balance. If you're looking for a challenge, you might want to give this a try. The update to this patch provided by David Gaider corrects Demogorgon's "beguiling gaze" and his domination of any demons the player should happen to gate in.

Improved Tougher Illasera greatly increases the difficulty of this first Throne Of Bhaal encounter, adding to her skills, weapons, AI and the strength of her group.

David Gaider adds: "It is not very high difficulty.. As a battle right at the beginning of the game shouldn't be.. But it does make Illasera a credible opponent and member of the Five. She's a ranger/archer now, which was her original concept before we were forced to make the battle much, much easier (since people could potentially meet her with a single, newly-created sorcerer.. And since the fight was unavoidable, those who couldn't get past it would be quite frustrated.) This version makes it so that you always meet the tougher Illasera as well as all her companions, so those who go into ToB with a weak character might want to stick with the original version."
Note: If you are soloing with a newly created character with few abilities, she might prove overly difficult.

Improved Tougher Gromnir adds new creatures, AI and abilities to the battle with Gromnir Il-Khan.
In David Gaider's description: "It's just a little spruce-up to a fight that I thought was a little boring.. It makes Gromnir a bit more formidable and gives both him and the other Bhaalspawn with him some extra abilities and better scripting. I'm told it's of moderate difficulty but quite fun."
The bug with Gromnir fleeing the scene has been fixed, as well as the problem caused by the fix file (which may have been corrupted in transit) where Gromnir just stood around.

Improved Tougher Yaga Shura improves the size and variety of Yaga Shura's army, Yaga Shura's toughness and AI.
As David Gaider's put it: "This mod makes the last fight with Yaga-Shura a bit more of a tussle.. the army spawns have been increased in number and given better AI (as well as had some low-level mages and clerics thrown into the mix). Yaga-Shura himself has been given some better powers and AI, including his lieutenants.. and his invulnerability slowly wears off during the course of the battle with him, making this fight a bit trickier. One caution: a couple of times crashes were reported soon after Yaga-Shura reappears. I was not able to verify if this was due to a specific bug or due to too much graphic overload, but it does seem to be rare regardless."
Note: if you have to play BG2 on "Minimal" setting, this may be a bit too much for your system. The update for adds two of the script files from the Improved Gromnir battle shared by this one which were changed (and debugged) by David Gaider.

Improved Tougher Balthazar was created by the request of experienced players who wanted this battle to be more difficult.
David Gaider elaborates: "Basically, this is just an optional patch for those people who found Balthazar too easy. Don't know how much tougher this makes him, but at the very least it's more fun. Balthazar uses his abilities more often and teleports across a farther range...and his pupils that fight with him have been buffed and each given one of his special abilities." The AR5505.BCS file has been removed from this update as some people were still having problems with game continuation post-battle even with the debugged version; if you have an older version of this update, simply delete the AR5505.BCS file from your Baldur's Gate II Override folder.

Improved Tougher Abazigal adds new creatures, spells and AI into this battle for high difficulty.
David Gaider's commentary: "Some of the posters were recommending a multiple-dragon battle, so I added in some smaller drakes and a purple dragon into the Abazigal fight. The AI has been tweaked heavily as well, blending in the mobility of Draconis (whom many thought was tougher than his father). It's quite the fight, now.. NOT for the faint of heart.. but I'm told it's quite fun."
Note: if you have to play BG2 on "Minimal" setting, this may be a bit too much for your system. Dave's v1.1 of this patch changes Abazigal's AI so that he won't Tail Slap or Shockwave during a Time Stop.

David Gaider's Disclaimer:
While provided by a Bioware employee, these files are not to be taken as part of the official release of Throne of Bhaal and should be considered equivalent to a third-party game addition or modification. BioWare does not guarantee that these files will work with your copy of the game. They are being provided solely due to requests for game enhancements, such as to experience certain game elements before they were removed or changed for game-balance or other reasons.