Prince of Demons Spoilers, of course...
St. Kargoth appears to be on foot, but he has a high ride bonus and various mounted combat feats. Is this accurate? I suppose the argument would be that he used to be a paladin, used to have a warhorse, and at that point would have selected these feats. Did I miss his mount? If not, I'll just retrain him with some help (and feats) from the PHB2. I want this to be a really scary encounter (possibly the climax, as I'm sure my PCs will want to grab the pearl, frame somebody, and get out before demogorgon attacks), and want him to be able to bring everything he has to bear on the PCs.
Well, there are two options I can think of for addressing the lack of a special critter to ride around on. First, cashier his mount (see the options for such presented in PHB 2 as well as a few Dragon magazine articles and the various splatbooks) and retrain his Ride skill ranks and any pertinent feats to more useful means of doling out carnage.
Second, depending on the interior description of the fell temple to Dagon in question, assign him an item to summon a high-CL version of phantom steed 2 or 3 times per day from which to sow discord and havoc amongst your player characters.
Depending upon how nasty you wish to make his critter, there are if memory serves one or two higher-level 'mount' type spells which should prove sufficient. And of course, the Goat of Terror Figurine of Wondrous Power could also prove devastatingly capable as a mount as well.
Prince of Demons Spoilers, of course...
St. Kargoth appears to be on foot, but he has a high ride bonus and various mounted combat feats. Is this accurate? I suppose the argument would be that he used to be a paladin, used to have a warhorse, and at that point would have selected these feats. Did I miss his mount? If not, I'll just retrain him with some help (and feats) from the PHB2. I want this to be a really scary encounter (possibly the climax, as I'm sure my PCs will want to grab the pearl, frame somebody, and get out before demogorgon attacks), and want him to be able to bring everything he has to bear on the PCs.
I saw the Black Unicorn in the Monsters of Faerun book. Part of the description talks about how evil wizards are working on a way to teach them to cast spells without the need for somantic components. Advance the BU and give it a few levels of Sorcerer or Wizzy, not to mention the Fiendish template (and/or others as appropriate)
Maybe a Fiendish Draconic Black Unicorn Sorcerer Dragon Disciple or something.
Regarding using the riding feats, the room that St. Kargie is encountered in is pretty large but if you want/need it larger I think the map can handle a change of scale without too much trouble. Then you can put those feats to use.