
Daspolo |

I am running a campaign wherein the pcs must fight their way through paragons of each of the chromatic dragons, and they are currently dealing with the torturous and sadistic black. The next will be the green dragon.
The green dragon lives in a dark wood of incredible corruption. It is filled to bursting with haunts and evil fey, among which is a dreaded whisperer. All these combined make the forest itself almost impossible to penetrate. Should they succeed, they will find the citadel of the green dragon.
The pf book, dragons revisited, describes green dragons as scholarly, constantly seeking physical and mental perfection. I would like to exploit this fact.
What would be fitting things a green dragon would desire to perfect itself. It can be magical items, class abilities, pacts, 3rd party, anything.

David knott 242 |

Let's see:
Advanced template: +4 to all of its stats could be appropriate. Alternatively, if you want to reward your PCs for killing it, give it a belt and a headband that grant +6/+6/+6 to physical and mental stats, respectively.
+1-3 sorcerer levels: To give the dragon access to 9th level spells.
Spellbook (taken from a relatively high level wizard) and Robe of Arcane Heritage: Once per day, this dragon can cast any sorcerer/wizard spell. Also provides PC rewards for killing it.
I am sure that there are plenty of other possibilities, depending on what form you want this "perfection" to take.

Daspolo |

The idea with the items belt and headband are both good ideas, except for my campaign using abp, but that's a good concept.
I think it having additional sorcerer levels would work.
I was kinda hoping to have his perfection be an academic/enlightened situation. Green's are supposed to be scholarly and someone isolationist.
I was tempted to go with some monk template, but that might be a bit silly. I was tempted to use some sort of psionic class as another avenue, but I already plan for the blue dragon to be the psion character so that's also being a bit back burnered.
I might give him a bunch of shadow piercings and demonic grafts, thought I may save the demonic stuff for the red dragon.
One idea I might go through is him using necrotoxins and similar stuff on himself intentionally. Like drinking ghost syrup to intentionally become permanently incorporeal.

avr |

Green dragons IIRC want to dominate others and have others serve them. You can see it in half of their SLAs: Young—entangle; Juvenile—charm person; Adult—suggestion; Old—plant growth; Ancient—dominate person; Great wyrm—command plants. I don't know why the Classic Dragons Revisited book has apparently rewritten them as scholarly isolationists. At the very least he should have a majordomo or other primary servant, and depending on how much you're giving out something like cult leader mesmerist might be appropriate.

Meirril |
Describe the plants of the forest as "explosively vital, as if the plants were being forced to rapidly grow, mature and wither in an unnatural fashion."
If the forest is observed from above at night visible strands of green verdant energy are seen streaming from the forest to a central point. That point being the Green Dragon's citadel.
Within the citadel the dragon awaits in his massive laboratory. The entire chamber is dimly lit, but the dragon himself stands in a spot light projected from the four corners of the room. The projected light is the final destination of all the positive energy collected from the forest.
The beam of energy heals any creature standing in it for 10 points per active beam per round. The dragon receives 25 points of healing per active beam because he has 'perfected' his own body by implanting ioun stones that allow him to more efficiently absorb positive energy and draws the energy to him.
When 1 beam is active the dragon heals 25hp per round, and will heal from death as per regeneration. Two beams only adds an additional 25 points of healing. While all 3 beams are active, the dragon receives the full benefit of a heal spell each round. With all 4 beams also remove negative levels, drains and make the dragon immune to aging and death effects.
Each of the beam projectors has hardness 15 and 100 hp. Each device can be disabled with an appropriate DC, or manually controlled with a DC 5 higher. If the beam is directed within 20' of the dragon, the beam is visibly bent to affect him. Darkness and Fog/Smoke have no effect on the beams, but solid physical barriers will block the beams. Force effects do not block the beams, and the dragon may use a force wall to guard a device.

Yqatuba |

Is the dragon himself already corrupted? If not, you could do a twist of having it be a rare good chromatic dragon who's acutally trying to cure the corruption in the forest. If the PCs take too long to get to him he might get corrupted and turn evil and they would have to fight him instead (is there some kind of "corrupted" template anywhere?)

SheepishEidolon |

is there some kind of "corrupted" template anywhere?
Horror Adventures has numerous, actually.
I could see the forest being twisted by the Shadow Plane, or the green dragon trying to perfect itself with artificial body parts, resulting in the promethean corruption. Accursed or hive might be even better fits.
Personally I'd stick with the themes of the area. The dragon might already be incorporeal due to its twisted strife for perfection. Or it gained some fey specific powers like quickling speed or natural invisibility. Given that players notice only so many things, I'd go with just two changes, but make them obvious and significant.