Motivations to have a secular wold versus a mystical world?

[Bakenal]

I'm having a bit of trouble coming up with a grand scheme for the conflict in my world. The setting is the classical ancient world. The armies of Alexander the great are grappling with his death and the empires are preparing to split. But the party will soon learn what Alexander's whole goal was. The land I have presented the group with is mostly secular with only a little in the way of magic and mystical monsters, far different from the ancient times of Greek gods and heroes. Alexander had been striving to return the world to the age of heroes by opening several gates dedicated to the gods. What I'm struggling with is his opposition in this. I have it so that dragons and other monsters like mind flyers and yuan-ti are striving for a more secular world where they can eventually defeat and enslave the sentient races. But how would you style this opposition? Should they know about the gates and actively moving against the party? Or should they utilize cults and organizations. But them what could those organizations motivations be?

[ThePurple]

But how would you style this opposition? Should they know about the gates and actively moving against the party? Or should they utilize cults and organizations. But them what could those organizations motivations be?

The answer to this depends upon how apparent you wish the interactions of supernatural creatures to be.

It might be that the supernatural creatures need the world to become more mundane (which is a better term, in my opinion, than secular, which specifically refers to a lack of religious influence as opposed to any supernatural influence) in order to make it easy to conquer, but their very presence among mortals would make the world *less* mundane. As such, they would need to act through shadow organizations, so that they can influence the world to make it less of a threat without their presence undoing everything they're attempting. Essentially, the gates aren't really what makes the world magical; it's the recognition caused by the gates that the supernatural is real that allows mortals to access it.

Conversely, mortal access to the supernatural could be tied exclusively to the gates to which you refer while supernatural creatures have innate access to it. As such, the supernatural creatures could directly intervene without acting counter to their purposes. In fact, by the very fact that they possess such abilities, it would probably behoove them to act in a direct manner rather than attempting to operate in the shadows since direct action is significantly faster. Perhaps, initially, they acted in the shadows in order to prevent mortals from rising up against them, but the actions of Alexander have forced them into direct action (which might be exactly how Alexander died; perhaps he was poisoned by the dying bite of a yuan-ti warlord that he engaged in single combat at the base of one of the gates).

All of it depends upon the exact mechanisms of the potential change in the world.

[quinron]

The only reason humans are dominant in the world is that we have superior reasoning power; in a direct physical confrontation with almost any wild animal, with no tools to use, we'd be killed or at best be mortally wounded while killing the animal. The monsters you mentioned all have at least equal brainpower to humans but are much more physically dangerous, so I would guess the reason they're trying to seal off magic is that they don't have access to it while humanoids do. If you send four appropriate-level fighters, without magic items, against a yuan-ti abomination, they're going to lose, every time; if you swap just one fighter for a wizard, their odds have increased exponentially. Without magical help, an army of 10,000 human soldiers is a sentient pincushion; a dragon is still a dragon. (http://www.giantitp.com/comics/oots0627.html)

The definition of "magic" among your monsters is important here. You could call a dragon's breath weapon (and basic ability to fly, pursuant to the bumblebee problem) or a beholder's eye beams "magical," but calling them supernatural abilities as they tend to be in 3.5 helps. I'd definitely avoid having genuine spellcasting among your monsters, though; it's a bit of a plot hole if mind flayers are trying to cut off all magic, but they have sorcerers in their ranks.

[Aedilred]

I think the first thing to do is probably to nail down the mythology of the setting to explain why these creatures exist. This gave me pause, for instance:
The setting is the classical ancient world. ... dragons and other monsters like mind flyers and yuan-ti

Why do these creatures exist? Who created them? Why are they present in a world otherwise similar to that of Greek myth?

If you can answer those questions that might in itself give you some inspiration for their motivations in trying to prevent the return of the Olympian Gods. And if you find you're just including them for the sake of it, perhaps that gives you the excuse to do some trimming.

Perhaps the laziest, which is not to say the worst, motivation could be that these guys are working for one of the traditional antagonists of Greek mythology: Titans, Giants, even Gaia Herself. But possibly more plausible is that most of them have differing motivations and causes, and all they have in common is their opposition to the party's plans. They wouldn't even necessarily have to be working together. It might be worth investigating broadly contemporaneous religious movements like neoplatonism and the concept of the Demiurge or (especially, given Alexander's relationship with Persia) Zoroastrianism and seeing if you can find any inspiration in there. Alternatively it could simply be in these creatures' interests, especially monsters who are innately powerful, to separate humans from their gods.