Ok, I didn't have the best day as those go, and it certainly won't let me sleep (it is about 4:15 in the morning here). But I've come up with some more things, they might be weird, they might be disturbing, but they might help as well. Anyway, you've been warned
So you want to perform a Darkhallow. One of the most important thing, as it seems, is the atmosphere. An area of fear and despair around the place where the Darkhallow will be performed. So let's make one of those.
At first, I would start with some sort of headquarter. Always good for the bad guys to have a lair. I like lairs.
For what you are trying to do, I would give them a church. Either they bought a church (Some are being sold, as far as I understand), or they stole it. How do you steal a church you ask? Easy: A big and old church (think along the lines of St. Marie of the Angels, Harry's favourite church) is being completely renovated. The bad guys sneak in and take over, and redesign it for their purpose. To give you an understanding of the degree: If you would look at the church in the nevernever, you would see the church, simple as that. After the "renovation" it still looks like a church, but it is build from bones and rotting human flesh. Inside the real church, you would find anything that any person of faith would think of as an abomination. You can get really creative here
Then there are the Knights of the Cross and the swords. Don't take them out. Use them. Use them hard. Ahm ok, to clarify: The Knights are a great force of good in the world, so what better to lead it into despair than to kill a few of their greatest heroes? Oh, and unmake the swords in the process, of course.
You might not want to do it with all 3 of the Knights/Swords, because that would feel a bit repetitive, so 2 of them can be killed off stage, while the last one (and make sure they know the last Knight IS the last Knight) will be killed in front of the PC. With his own sword.
If the necromancers want the nails, they are inside the 3 Swords. As the necromancers are human, they should have no problem to unmake them and retrieve the nails. Even better: If the swords are not only named "Hope", "Faith" and "Love", but are also like conduits to those emotions, unmaking the swords in a big evil ritual could backlash into the surroundings and enforce the desired atmosphere needed for the darkhallow. You are not only going to get a few more sessions out of this, but losing a Knight and actually seeing the world around them make a turn for the worse should be enough to make them understand how dire the situation is. I know, this is a little railroading (they have no chance of saving the knight or the sword. Ok, maybe they can save the knight.), but totally worth it, in my opinion. There could even be spies in the churches organisations connected to the knights, that betray them and lead them into traps.
During the whole story, you should kill of a bunch of the necromancers. Either you throw them to the wolves (also known as "your players") or they get killed by their co-workers. A victory every once in a while is a great way to keep players motivated in an otherwise hopeless scenario, so don't take away that fun for them. Of course they can be victories like "they kill one necromancer, but the other gets away with an artifact" or "..., but he can kill the knight of the cross" and so on. Make them suffer
Then there is the whole Jesus issue. It might be possible to summon 2 Jesuses. Jesii? Ok, it shouldn't be possible for grammatical reasons, but let's not get carried away by that. Jesus died at the cross, and he... kinda... died again. So maybe it is possible to summon one Ghost-Jesus with the Cross and the Nails and everything and the other with the Shroud. If it is possible, you could have 2 darkhallows at the same time, maybe the PCs can find a way for them to cancel each other out. One necromancer sucking the power out of the other and vice versa. I like that picture.
Of course, a single Jesus would probably be better, but I wanted to mention the idea above anyway. If you go with the single Jesus, the crucifixion scene would be a great way to go. The church should have some sort of garden or park close by, that would be the best place for something like this. Of course the plants are mostly dead, fungi and the likes spreading over formerly green grass and tall trees. When the ritual begins (which should be about when the players arrive) the items are placed along the line of the crucifixion walk. Shadows appear and form the loose shapes of roman townfolk and legionaries. The necromancer (at this point he and the players should have wiped out the competition) is standing at the end of the walk, next to the cross, ready to nail Ghost-Jesus to the partly-ghost-cross. If this is completed, the real darkhallow begins. The most interesting part is going to be during the crucifixion walk. The closer Ghost-Jesus gets to the cross, the more corporeal not only he himself but the whole scene will become. If the Players start interfering, the townfolk and legionaries will attack them. Not because they are ghosts controlled by the necromancer, but because nobody will stop roman justice, it is their duty as good Romans and legionaries.
If something was to wobbly-bobbly in there, I will be glad to clarify, just ask. I hope you get a great game out of this. Enjoy