How should I roleplay an extremely powerful sorceress who has just come into her powers?

[slachance6]

One of the major NPCs in my upcoming game is a previously ordinary person who has recently been imbued with the immense arcane power of a forgotten archfiend. She originally lived as a middle-class resident of a relatively civilized human kingdom, and is somewhere from late teens to 20s. Then she discovered her powers and opened a demonic portal. She doesn't fully realize the extent of her powers yet, and certainly doesn't know the origin, but knows that they are somewhat fiendish in nature, and absolutely knows she has mind control.
I haven't quite fleshed out her backstory yet, but I know she is an enemy of the nation's leaders (she hasn't sworn vengeance or anything, but she doesn't like them). She's also generally overconfident, and hates in when people consider her inferior to them, even if it's true. Due to this, I think she might just try to take over the kingdom, in which case she'll probably succeed.
Only a few people know about her powers, and they will try to manipulate her into using this power for their own purposes. Most people do know that some power has risen, though, and a small cult has formed. There are also clerics and paladins who believe that she will be corrupted by the fiendish nature of her powers (which she won't, at least not through magical alignment change).
To give an idea of the power level (this is a Pathfinder campaign but I think this topic can apply to fantasy games in general), I took the succubus monster (remember, her powers are fiendish, and she specializes in enchantment), added 20 levels of eldritch godling and mythic tier 10 (basically the most powerful creature on the Material Plane), along with plenty of magic items that I'm flavoring as innate abilities. Her final ability scores are 19, 22, 24, 24, 22, 50. She's is particularly talented with charms and polymorph (both self and baleful). Her character sheet is here on Mythweavers: http://www.myth-weavers.com/sheet.html#id=733359
So, how would this character react to becoming akin to a minor deity? How would her personality change if she discovered that her commands were invariably followed, and that she could conjure fireballs and balefully polymorph the population of an entire city? Bonus points if you say the outcome for each different alignment (I haven't decided yet, but it probably won't be lawful).
How should she generally act when the players approach her? They are level 10 and have heard several rumors, and are going to meet her at a secluded location in the wilderness that she has recently gone to try to get used to her powers, hoping to ask a favor of her that she isn't opposed to, but doesn't particularly care about. What twist should I add to the scenario so she doesn't just blast them to ashes as soon as they mildly offend or annoy her? What do you expect the players to do? What do you think she would do in the future?

 

[Celebrim]

Start singing, "Let it go, let it go, I can't hold it back anymore..."

 

[VikingLegion]

This sounds a bit like the old Dark Phoenix arc back in the late 70s and early 80s X-Men comics. Jean Grey is granted with this nearly godlike power. She struggles to adjust but basically tries to remain true to herself and use the power sparingly, and for good purposes. But of course the temptation is always there, and as her powers rise so to does her proclivity for quick and expedient ways to resolve problems (i.e. increasing cruelty). There's an inverse relationship with her power and her "connection" to humanity - as the former ramps up the latter declines. Mortal minds just can't handle all that power, especially when it is granted so swiftly and not earned over a long period of time. I see her as setting out with only the best intentions: taking over the kingdom because she can "do it better". Her newly imposed utopia will start off wonderfully - food for all the peasants, security from potentially hostile neighbors - a (short lived) golden age of prosperity. But once the luster starts to wear off and the people chafe under her rule, well that's when good folk suddenly go missing and thought police patrol the streets, beating or arresting anyone that doesn't drink the Kool-Aid.

As for the last part of your question, if they do meet in the clearing and they offend her, have her initiate a "blast them to ashes" attack, but then maybe pull back at the last second as her waning humanity asserts itself just long enough for her to tell them to run. If you want to really mine from Dark Phoenix she could then scream at the players to kill her before her "other side" regains control. If there's a certain type among your players he/she may just try to strike right then and there to end the threat. If that end is acceptable to you, let them do it. If you don't want to let this NPC be offed quite yet, say that in the instant before the blade strikes home a dark smile twists her face and the player is hurled back in a concussive burst. The sorceress-demon then laughs at the players, deems them unworthy of her attention, and then flies off to go do something awful, leaving the players bewildered in the clearing and wondering what she will do next.