How to kill an unkillable Vampire

Three words: illusion of choice. My players have almost always gone in the direction I planned for from the start, and I have never ever received a single complaint of railroading, even when I've asked if there's any issues with the game. It helps that I have a really good group of players who tend to build characters they really get invested in, and apparently my strongest point as a DM is knowing the PCs well. Plus, the party are already on the quest that leads to this in the end, and have been since session one. This whole thing is what the party was put together to do.

They just don't know it leads here yet.

Hmmm, this is doable I think. Light can be done with paladin holiness stuff, pools can be done easily. Magnetising may be done by the dragon (dragons in the setting munch on certain minerals to use their fire breath, then supplement it with magic, and dragons wind up knowing a hell of a lot about geology, and it would probably wind up being the dragon that thinks of the magnetism idea).
Just need to think of how to describe the whole culmination of being bloodgod thing. I know that her gemstone necklace with her tie to the rest of its being will be torn away and flung outside of the baths. Could have it that his sword becomes blessed by his god during the fight with the essence of the blood god.

> Paladin uses holy power to create holy light from the walls and pillars of the bathhouse they are fighting in. This makes the vampire's shadow forms impossible to use, and prevents escape via shadows
> The pools in the bathhouse are tiered at different levels. Dragon enters the bathhouse by breaking the walls at a lower level tier. He uses an artefact from his long was with the merfolk to open a portal to the ocean at the highest tier, after smashing up some of the flooring to create pathways to lower tier pools to make running water, prevent escape as non-shadow. Can't cross running water.
> The dragon uses its magic to magnetize the iron in vamp's blood to keep her still
> The dragon uses its magic to allow paladin to travel to vamp's personal demi-plane, where he has to fight the essence of the blood god within her.
> He fights, and truly gains the blessing of his god.
> Gets holy godsword gift
> Apply godsword to bloodgod
> Paladin uses both hands to stab vamp's heart with the godsword. Vamp does not die, but is rooted there, but only whilst paladin keeps her stabbed.
> He gets dragon to melt rocks into lava whilst he is there
> He uses his power to sanctify lava until it stays lava under it's own power
> He then gets dragon to drown the both of them in holy lava
> Paladin dies
> Vamp dies

Thoughts?

Launch her into space.

Inherently silly.
Does the OP have to kill off/retire the paladin?

Pretty good. Nice, concise, doable by a decently high-level party and worthy of song. Bonus points if...

Eins: the demiplane gateway is still open, so some lava gets in there, too. Could make for an interesting plot point, if her bloodgod-bits end up fighting with holy lava....things stuck in her demi-plane, playing merry hell with her powers until she deals with that problem.

Twei: The paladin dies standing up, as if standing guard before the vamp's remains. So that if the player's plot kicks off with our vampire getting unearthed and somehow resurrected, while the researchers either fail to notice or are prevented from unearthing the (now full-on saint of vampire hunters) paladin's remains to easily solve the problem.

If not in this fight, then later. He's shown up in the previous campaign as an intelligent item in the form of the Paladin's tower shield.

Silly humans want vampires which surprises them given they behave the same as supernatural entities.

stakes made out of fresh wood are sometimes required to kill or disable vampires effectively. Fresh wood doesn't burn as easily as old dried out wood.

Is this 3.X D&D, because if so, 5th level HUNTER OF THE DEAD.