Need help plotting a dragon encounter

[shrewd bane]

Hey guys, so need some input here. My PCs finally reached on a long game I had been playing out and thankfully it seemed to have gone over rather well.

The setup

I had the party periodically interact with an elven woman who was at first a chance meeting but then upon subsequent meetings proposed a deal that the party refused. This woman is a greatwyrm who lost her children to a Kingdom who periodically hosted Hunts where they hunted rare monsters for sport and show off their power.

She wanted revenge and since the party didnt help her, she created a Conclave or Council, where each member of the group joined only because they stood to gain something from it. They are now causing havoc all over the continent with the PCs also being affected and trying to solve it since they have found themselves in the middle of it. The green dragon is the only greatwyrm whereas the others are old adult or ancient dragons.

Long story short, there is a black dragon who lives in a different continent that joined simply because it is rumored that it is obsessed with magics and destroying a city state in her home continent that has been causing issues but she has so far been unable to do so, thus she agreed to get access to tomes kept in the premier magical academy of the continent the campaign is set on. Black dragon is infamous in their continent and is usually left alone as any attempts at attacking her domain has left no survivors.

The PCs have met with the greatwyrm and are now trying to turn her against the others. I have stuff planned out more or less but I do have a problem in the sense I have three routes for the black dragon and I cannot decide which one I should go for so here I am asking for thoughts.

Routes

1. Black Dragon is a puppeted by the green dragon and has been mind controlled by the greatwyrm some time ago, and the PCs can either break the mind control (if they find it out) or kill the black dragon.
2. Black Dragon has been dead and used as an undead entity by the Green so as to keep tabs in the Material Plane while she lives in the Fey Realm.
3. Black Dragon is an independent entity and the only way to combat her is either to give her what she wants or kill her.

I want to try to have a "plot twist" though I admit planning for a plot has never worked out well for me, hence this post in seeing what others think of it.

Please let me know if anyone wants more info

[finite tapir]

Could you reword your second to last paragraph? I’m struggling to understand who is doing what and why.

[shrewd bane]

Under routes or the context?

One of the dragons part of the conclave is an ancient black dragon, that rules over a large territory on a different continent and has been known to previously attack a city state north of its lair but has not successfully destroyed or conquered it, which has not done well for its ego. It is also a mage and is ever hungry for more tomes. It's reason to join the conclave is to get tomes from the continent the campaign is set in.

• The city state has tried sending people to kill it, but none of them have returned.
• In terms of difficulty or powerscale: the green greatwyrm is the strongest in the conclave followed by the black dragon.
• The Green dragon approached the other dragons playing on their desires.

[finite tapir]

Then I think it being independent with aligned goals is appropriate.