Kamikaze Midget said:
But since it's a D&D adventure, I'm going to need a way for the players to thwart that plot. This means rules for things the succubus can do outside of "beat the crap out of the PC's."
You've got that, though. It's in her skills and her defenses. Or more to the point - the PCs can be assumed to succeed unless there's some sort of adversity. The amount of adversity the succubus supplies is defined by her skills & defenses.
Instead of having to come up with a way for the players to defeat the succubus, you have rules for how the succubus can resist their attempts. You can just sit back and respond to their actions.
Kamikaze Midget said:
I'll also need ideas for future adventures, ideas for how to make this unique, and something to make a succubus "seduce the king" plot different from a random high-Charisma NPC "seduce the king" plot.
A succubus is a random high-Cha NPC. Except that, since she features in the game, she's not random, and she's an evil demon.
The statblock isn't going to make it interesting - except that it needs to provide a necessary level of adversity. Characterization and thematic resonance will make the story rock. You have to provide that. What the rules can do is enable you to do that, by not getting in the way and by making her a fit adversary.
Kamikaze Midget said:
In short, I'm going to need more than "She's a sexy devil who is sexy and devilish."
I don't!
There's a lot of power in the succubus trope.
Kamikaze Midget said:
Two sentences gave my imagination all the spur it needed to run with it. Nothing in its statblock, less than in 2e, just preserved the coolest ideas.
Yeah, all you need is a line or two of fluff text in the MM entry and you're off to the races!
Kamikaze Midget said:
The succubus might be able to get away with it by force of pure cultural push: most people will be familiar with the idea of "seducer devil," and plots involving them are not rare.
But for whole-cloth D&D monsters, like the phane? There's no storied history of cat-centaur living weapons who shoot AARP membership lazers.
The phane is stupid. But: "Phanes can manipulate time, which they use to sow chaos among mortals. Occasionally they form pacts with powerful beings that share their destructive propensities." That sounds good enough to me.
Just from that, I can picture an adventure featuring a powerful noble dynasty with a reputation for doing good deeds. Now, something's changed. The head of the family is sowing chaos in a tense political situation (a war is at hand, or a King has died with no clear heir, or the change of a religious head).
The backstory is that a phane has just showed up and wants to mess with the current political situation, so it goes back in time, gives its support to an ambitious dude (better if he's a relative of a PC), and manipulates the dynasty over the years to shape it into the tool he needs right now.
Throw in a bunch of skill challenges that hinge on finding this stuff out, then a ritual to go back into the past, and there you go. You've got all you need.
I didn't look at the statblock to come up with that; I don't think I would need to.