Thank you for asking after Drendd and Bahamut. I'll go ahead and e-mail information to you regarding Bahamut, since he's not open content. Information on Drendd, however, I will post here. Based on what I post, if you have lingering questions, please do ask. I'm likely to be away from the computer all of tomorrow but may be able to get a post in on Friday.
Drendd is one of the Elder gods in my pantheon. Not every god is an Elder god...witness Narn, the Tinker, the Luckmaiden, etc.. No special in-game benefit will be accorded to PCs who worship from the Elder pantheon, though I note that this information may prove useful to you during roleplay. I've always envisioned the Elder gods as garnering more respect and authority amongst the younger upstarts of the pantheon and these relationship issues may, in some instances, translate to how clerics loyal to different gods treat one another while on the material prime. "Ohhhh, you worship Galarn? Yeah, bow down before my divine prowess given me by Othar, you worshipper of a non-Elder god, you!"
Many of the Elder gods have a familial connection to one another. Madriel and Othar are sister and brother, for example. In Drendd's case, he and Tinuviel (the goddess of vengeance) had a fling during which the Triune was produced. This is why the Triune's full name is Ilesere Tinuviel do Drendd. While Drendd is often regarded as dwarvish in flavor, I personally have always regarded each god and goddess as being faceless and somewhat cultureless--at least in the ways in which mortals tend to ascribe these qualities to people. To dwarves, Drendd is thoroughly dwarvish. Humans, however, also worship Drendd and to them he would maintain a more human visage (though probably he'd look like Father Christmas and have a long beard, at the least).
Temples of Drendd will always be built of stout stone. Drendd's places of worship will often be symmetrical, as this is pleasing to the LG mind within my setting. You won't find many shrines or chapels devoted to Drendd; if a place of worship is worth building, then it's worth it to do it in grand style. Drendd's temples will often have a library and a resident historian.
Narn is also one of Drendd's offspring, but unlike the Triune, Narn was not produced of a physical coupling. Narn is partially analogous to the way in which Athena was produced asexually from Zeus. Narn sprang fully formed to life, armed and armored, from Drendd's thigh. To the great dismay of his father, however, Narn is hasty, quick to anger, and always ready for battle. Where Narn cannot resurrect or raise the dead back to life (because to do so is to do dishonor to battle), Narn is adroit at healing. After all, it's no sin to prevent someone from dying--even though it would be blasphemy to bring them back to life after they've fallen. Followers of Narn, therefore, are exceptional healers. In-game, all their cure spells are automatically maxed. Drendd has a few tricks up his sleeve that Narn doesn't have, however. Drendd grants his followers no special healing prowess other than what is normally accorded clerics, but his followers are not prevented from raising folk from the dead, etc.. I've always imagined Drendd stifling an inward smile of petty satsifaction whenever a Narnite Pardoner brings a dead body to a Drenddite Pillar, begging for special dispensation in the form of resurrection. Kind of goes with the whole antagonistic father/son relationship that Drendd and Narn have.
Those are the basics of what I have in mind for Drendd. Again, if the info above doesn't answer the questions you have, please bring your specific question(s) to my attention and I'll see if I can do better.