Hi everyone, I'm new here so apologies if this is a frequent question and I'm beating the dead horse, so to speak, but...
I'm thinking of running a Planescape campaign. I understand Asmodeus' origin, nature, and power is much greater in PS than it is in base 3.5 edition? What book or books should I read to bring me up to terms, or if someone can give me the basics, that'd rock too.
On a similar note, are there any other deities or powers from 3.5 that take on greater importance or majorly different origins in Planescape that I should be aware of?
Sorry, shoddy memory... of course I meant "Guide to Hell", not "A Paladin in Hell"... but as reguards Ahriman and Jazirian being two creators of the Great Wheel, ever wonder about the third? I mean, rule of three and all... maybe the Lady? (It would neatly account for Her alledged Lawful Neutral alignment, if this theory is accepted... after all, Jazirian is Lawful Good, Ahriman/Asmodeus is Lawful Evil... but perhaps the story is a lie spread by Asmodeus himself to inflate his importance in the cosmos, and to unnerve his foes! In any case, neither Jazirian nor the Lady are talking...)
Asmodeus's origin, nature, and power are left very vague in Planescape (the Dark Lord of Nessus isn't even named in Planescape materials). The power of the Lords of the Nine in general is vague - they have avatar stats, but no stats for their true forms. It's suggested that they might be the intelligences of the layers of Baator itself, or primordial law-tainted yugoloths, or simply ascended pit fiends. Or a mixture thereof. Bel is definitely an ascended pit fiend, and Triel (Baalzebul) is definitely a fallen archon.
However, it's clear that they're on par with the gods, at least while in their own domain. The least of them, Bel (or possibly his predecessor) is credited with possibly driving the entire Orcish pantheon out of Baator and into Acheron. Levistus is at war with Set and Sekolah, and neither deity has yet been able to destroy him. On the other hand, Levistus hasn't been able to destroy either of them, either, or drive them out of his layer as it is thought Bel might have driven the orc gods out of his.
All the Lords of the Nine have great powers over the natural forces of their homes, able to direct the elements at their command and, if sufficiently moved to concentrate, they have a limited form of omnicognicance within their domains, able to know all that happens there if they do nothing else. These powers don't extend into the realms of gods who dwell in Baator, however.
The problem is that gods in 3rd edition were given extremely powerful stats, well beyond what mortal PCs were expected to achieve, while archfiends were generally slotted into a range where powerful mortals could kill them. In Planescape, both gods and archfiends are generally beyond what the PCs are expected to be able to do. This is similar to their status in 1st edition, when all archfiends were the equivalent of lesser gods in power. 4th edition also puts archfiends at a power level where one, or sometimes a group of them, can kill a god - and several of them have been said to do so.
But without stats for either the Lords of the Nine or the gods in Planescape, it's difficult to say how powerful they are with any precision. I'd say the Dark Lord of Nessus is certainly powerful enough that Kurtulmak can't stroll into Malsheem and take over, something that isn't true in 3e, where Kurtulmak's statistics could easily wipe the floor with Asmodeus no matter how many troops he brought to bear.
The relevant texts are Colin McComb's article in Dragon #223, Planes of Law, On Hallowed Ground, Faces of Evil: The Fiends, and Hellbound: The Blood War.
Oh, and as far as origins go, there are some definite differences. The ur-myth used by Colin McComb in Faces of Evil and Hellbound involves the General of Gehenna creating the other fiendish races by casting off the taint of Law and Chaos from the yugoloths at the dawn of time; this taint became wriggling larvae, who evolved into tanar'ri or baatezu depending on whether the taint was chaotic or lawful. This is pretty different from the obyrith creation myth in Fiendish Codex I and the "fallen angel" creation myth in Fiendish Codex II. The difference isn't insurmountable, though, if you assume that it was the obyriths and ancient Baatorians who were created by the 'loths rather than the tanar'ri and baatezu; and, indeed, Fiendish Codex I and a few other sources suggest this may have been the case.
As for other differences, I can't think of any at the moment. Except for a few issues involving modrons or quasielemental planes, 3rd edition generally built on Planescape canon rather than contradicting it. Just remember that the Lady of Pain isn't lawful neutral, as the Planar Handbook claimed - no one knows what alignment she is.