
james014Aura |

I'm working on homebrewing a campaign where the PCs are purifying blighted land in the First World, but I'm looking for a creature that could be a root cause of the problem, but also would be reasonable for a high-level party (but neither epic nor mythic) to fight.
The main thing I can think of right now is Vorgozen, which is ... a CR 29 Kaiju. That feels a little tough for the party, even if I give them a liberal helping of high-level Fey allies and an army.
Meanwhile, Hezrou and Omox demons feel a little too weak for the campaign.
Ideally, I'm after something associated with Pollution, Curses, or Undeath (or more of the same), and suited for a level 15-17 party (I can add minions if they're of the right flavor).
(I can make it a Siabrae if needed, but I'd rather have that be a last resort for what to make the end boss)

Ventnor |

How about you start with a wild hunt monarch and add a template to represent it being corrupted somehow.
This corrupted monarch spread the corruption in the first place to create bigger and badder creatures for it to hunt down with its corrupted wild hunt hunting party.

james014Aura |

A Blighted Fey with Fungal Creature allies could do it, though I was hoping for the source of the corruption, not a simple spreader of it (who was in turn corrupted). Though, the raw evils in the main lands I intend them to explore would make up for that.
@GM Rednal: While I'm not sold on a Seilenos final enemy, I think you just pointed me at a potential ally for the part of the campaign where they have to keep purified lands ... well, purified. Though an uncorrupted Glaistig could be the end boss... or a partial cause to pacify and then recruit to fight something tougher. Thank you for pointing me at those two.
@Ventnor: I'd actually considered that exact monster as an ALLY. Though, Erlkings are almost the same thing, almost as powerful, and would probably be more thematic. Thanks for reminding me of that option.
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Thanks everyone for the ideas. While the Oracle won't be level 20, it probably will be a Dread/Cursed Lord who shows up as one of the lesser centers of the curse. And the others have given me the solution for how the curse happened in the first place: an abyssal rift forming from the raw evils of the Dread Lords (etc) causing Blighted Fey and Fungal Creatures.

Bodhizen |

I'd add in the ability for it to cast baleful polymorph a few times a day if you've got a bunch of druids playing (which I am guessing from the title of the post). That could be fun and force high-level druids to use up some wild shape uses for the day in fighting him. I'd give him no more than 1/druid in the party.

Louise Bishop |

I'd add in the ability for it to cast baleful polymorph a few times a day if you've got a bunch of druids playing (which I am guessing from the title of the post). That could be fun and force high-level druids to use up some wild shape uses for the day in fighting him. I'd give him no more than 1/druid in the party.
Or can burn the Bead of Newt Preventions that they should be carrying.

The Sideromancer |
One could easily say that construction is a natural state for organisms to be in. The particulars of a scorpions burrow are hereditary, a beaver is well-adapted to making structures (heck, there's been one visible from space), and there's a pretty clear reason why in PF Brigh has the sacred animal she does (termite).
As well, there's the evolutionary response to take advantage of any uncontested resource available. When there's magic and materials that allow one to predate a greater varieties of creatures, a hypothetical SLA-using creature with Cold Iron fangs is as much a result of natural processes as larger dragonflies using oxygen-heavier atmosphere or uranium-fixing bacteria IRL.
I'm biased against fey because they seem to propagate a view completely contradictory to my own: that the only nature is that "coincidentally" without their primary weakness.
TL;DR, if I saw it was a construct causing it, I'd probably break character and defect.

Reduxist |

How about an elemental made of cold iron that simply wants the right to exist? It's not his fault that he endangers others by simply existing. He was just naturally there; granted, his very existence was a threat to the first world, so they attacked him. In his desperation, he made contact with some... unscrupulous entities, and things snowballed from there. Sympathetic, but reasonable to take out, no?

The Sideromancer |
How about an elemental made of cold iron that simply wants the right to exist? It's not his fault that he endangers others by simply existing. He was just naturally there; granted, his very existence was a threat to the first world, so they attacked him. In his desperation, he made contact with some... unscrupulous entities, and things snowballed from there. Sympathetic, but reasonable to take out, no?
I would still like to deal with the root cause first, both the initial aggressors and the ones using the iron for less-than-savoury purposes.

Reduxist |

A Wyrm or Great Wyrm Umbral Dragon. They breath negative energy, are often allied with powerful undead, and can spawn shadows under their command. As a kicker, give it the lich template and make it the leader of an undead army/conspiracy/nation.
Speaking of undead dragons, how about an Azi, specifically an Azi Zahhak? Their Slain to Serve ability makes an excellent cannon-fodder generator.