Plan B.

[GenocideHeart]

Well, there was Harvey the Rabbit an invisble (except to the main character) talking rabbit, from a really old movie named predictably "Harvey" http://www.imdb.com/title/tt0042546/ ; also in Farscape, there was a Scorpius in Criton's head that he called harvey (presumably in reference to Harvey)

Ah, I see. This reminds me of a fic I'm writing based on Evangelion, heh...
Thanks for the explanation.

[Ozzallos]

...

[Questara]

This is NOT what I'm aiming for with Ranma, as it's rather cliche in fanfics... y'know, the "together my two mental aspects can reason with one another and find a middle ground, combine our powers and form wonder Ranma" plot.

For some silly reason, I just got an image of Ranma yelling, "By my minds combined, I am Captain Ranma!"
Hmmm, wonder if anyone will ever do a Ranma/Captain Planet crossover/fusion

[Screwball]

This is NOT what I'm aiming for with Ranma, as it's rather cliche in fanfics... y'know, the "together my two mental aspects can reason with one another and find a middle ground, combine our powers and form wonder Ranma" plot.

So is he regularly going to catch it when he starts thinking thoughts that aren't his, like at the end? Or is he going to be influenced without realising it, and go on a glowy eyed rampage every time he gets provoked?

For some silly reason, I just got an image of Ranma yelling, "By my minds combined, I am Captain Ranma!"

Now I have an image of Ranma in a rubber suit with nipples on it, ala Batman, stuck in my head. Thanks.

Hmmm, wonder if anyone will ever do a Ranma/Captain Planet crossover/fusion

I'll hunt them down if they do create that atrocity.

[Eris]

Yeah, a "harvy" refers to an invisble, independent aspect of the character's personality only visible to that character. Normally the character can interact with the "harvy" as it attempts to influence their behavior.
This is NOT what I'm aiming for with Ranma, as it's rather cliche in fanfics... y'know, the "together my two mental aspects can reason with one another and find a middle ground, combine our powers and form wonder Ranma" plot.

Ahh, so that's what a harvy is. You answered my question before I could answer it. Incidentally it didn't particularly feel like one to me. I found it to be more of an internalised rationalisation process. Which brings me to my main point...
The entire thing felt a bit rushed to me. Ranma went from what I assume is a baseline behaviour to, well, vicious in all of one fight. I personally would slow down the process a bit, having the insights and rationalisations build up slowly over several fights. Two reasons basically.
Obviously one is just for pacing and plausability. Even with the hints of magical influence going on, it just seems remarkably not Ranma-ish to flip out like that. Having it start with, say, a slightly rougher treatment one day of Mousse for semi-justifiable reasons, moving through proggressively harsher and nastier for less purpose would make the entire thing smoother.
Also, and here I have to speculate for a moment on your overall intent, it would make the entire behaviour change harder to stop. A snap change like this is very obvious to everyone involved, perhaps even to Ranma upon self-reflection. Having it slower would make it a bit harder to spot at once, and moreover Ranma would have a much harder time seeing the change as unjustified. There would be a logical progression to her actions that she could fall back on to avoid any thought of reversion.
My two cents.
More generally though I see much promise here. Damn if we don't need more villainous fics anyway--there is altogether too little evil in writing these days.

[GenocideHeart]

What you say is true, Eris. However, others could simply see it as Ranma finally snapping, and ranma himself may see it that way. He lives a very stressful life, even by his abnormal standards, and he may well chalk his actions up to just reaching his limit.
Now, if it happens more than once, THEN he starts questioning it.

[Pale Wolf]

However, others could simply see it as Ranma finally snapping, and ranma himself may see it that way.

Barring, of course, the eeeeeviiiiil red eyes Which, conveniently enough, Ranma can't see.

[GenocideHeart]

Barring, of course, the eeeeeviiiiil red eyes Which, conveniently enough, Ranma can't see.

Well, Ranma can already glow in the dark if he releases his aura. For all anyone normal (or brainless) knows, the glowing red eyes may be a manifestation of rage caused by uncontrolled chi.
Now, an actual master, like Happosai or Cologne, will KNOW there's something very wrong with those glowy eyes. But Genma? Soun? Akane? Ryoga? They don't know nearly enough to tell what it is, and all of them may well dismiss it as Ranma losing control. Creepy, but what else could it be?
...if only they knew. Hehehe.
Now, if it were a Senshi... well, Ranma in that state is probably emitting so much Negaforce that he'd fry the Mercury Computer if it picked his energy signature up. So they could probably figure out what's going on... to an extent.

[Shadow Wolf]

Well, there was Harvey the Rabbit an invisble (except to the main character) talking rabbit, from a really old movie named predictably "Harvey" http://www.imdb.com/title/tt0042546/ ; also in Farscape, there was a Scorpius in Criton's head that he called harvey (presumably in reference to Harvey)

I think Sunshine was referring to Harvey Oswald, AKA Two-Face of Batman fame.

[SoftRogue]

I think Sunshine was referring to Harvey Oswald, AKA Two-Face of Batman fame.

Um...you got a little turned around there. Harvey Dent is known as Two Face while Oswald Cobblepot is the Penguin.
Truthfully, I thought that Sunshine was refering the concept of a 'Harvy Stu', the male equivlent of a 'Mary Sue'. In other words, the rediciously overpowered self insert cum existing character that basicly throws out every 'rule' that the story that the character come from and does whatever the wrighter wants while ignorning everything known about said continuity.
Of course, I could be wrong...

[Sunshine Temple]

No... SoftRogue and Shadow Wolf you're both quite wrong and Questara and Ozz are right.
now be careful you two with silly posts.

[Alathon]

I kinda like the idea of this story, and think it has great potential to be cool, but really hope it doesn't end up breaking suspension of disbelief in terms of what Beryl/Metallia are capable of and would do with Ranma's strength. There's a tendency in Ranma 1/2 fiction outside of bash-fics to have things "get bad" but end up having easy outs to avoid things like main characters killing or dying.
Of course, there's the very real question of what's going on with Ranma, which we don't know. What sort of deal did Nodoka cut... was her deal meant to be something benign that just ended up not being that way, or did she sell him out worse than Genma ever did? Does Beryl's consciousness still exist in the back of Ranma's mind, or was she perhaps making a bid for vengeance by her descendant. In which case it'd be Metallia's influence that'd be relevant. Which of course would raise continuity questions, since beyond a certain point in the SM timeline she should be ashes and dust.
You might wanna describe in detail just what part of his eyes were red, and in what way, if that's gonna be the hallmark of Beryl/Metallia's influence. Was it his pupils? Was it the entire eyes? Was it a glow over the eyes, or the sort of opaque red glow we usually see on inhuman bad guys and sorcerers casting evil spells?
The bit about his female form being sharper mentally makes a lot of sense in this context.. but would actually fit in any story, given the scenes you pointed out. And that physically, his female form could well have suffered far fewer knocks to the head.
One thing I think is worth keeping in mind, though I dunno if you plan to go there, is that Ranma does have the potential to throw in with Metallia. It wouldn't be easy to manage, but he has all the necessary faults of character, if suitably provoked. If he had a chance to be something more than what he has been, to achieve a level of mastery worlds beyond what he's seen in life, just how much prodding would he need to make a grab at it? Offering him power wouldn't mean that much, but for example, an opportunity to train towards mastery of life force manipulation orders of magnitude more complex than what he knows, would definately attract his interest. And while he isn't a whiny emo brat, it wouldn't take that much nudging to slant his view of the rest of the Ranma 1/2 cast. Though it'd have to be more subtle than in the last scene, which suggested actual possession.
There's also a middle ground between falling and being unchanged... being forced to grow up emotionally in the process of dealing with urges that any psychology text (or Kasumi?) would likely describe as entirely natural given his circumstances. Which would be kinda funny, an evil force attempting to corrupt him, and instead forcing him to mature to deal with it.
Just because Happosai and Cologne are old martial artists and have had their fingers in a few arcane pies, doesn't mean they'd know a damn thing about the Moon Kingdom, Beryl, Metallia, or anything else about Ranma other than what they've observed. Don't feel obligated to have them be in the know unless it's good for the story -- they're not Elminster, Gandalf, or Gorynel Desse.

[GenocideHeart]

Just because Happosai and Cologne are old martial artists and have had their fingers in a few arcane pies, doesn't mean they'd know a damn thing about the Moon Kingdom, Beryl, Metallia, or anything else about Ranma other than what they've observed. Don't feel obligated to have them be in the know unless it's good for the story -- they're not Elminster, Gandalf, or Gorynel Desse.

That's not what I meant when I said Happi and the ghoul would know something's wrong. What I meant is that they'd realize the whole 'glowy eyes' thing and the sudden spike in strength isn't ALL chi.
Now, identify it as Moon Kingdom origins? Hell no. Identify it as something that shouldn't be in Ranma? Yes, definitely.
They'll know that something is very very wrong the moment Ranma flips out anywhere near them. And they'll know it's not natural at all. Beyond this... I doubt they know anything besides old rumors and legends. Although, being over 3k years old, the Joketsuzoku tribe might well have some ancient lore regarding the Moon Kingdom.
Just my $0.02.

[Ozzallos]

...

[Screwball]

Oh dear. Poor Pluto. I wonder what she'll do about this? Can she do anything about this?