They get away enough that they won't need to worry about clothing damage even if there are large explosions or whatever.

The area the dragon is assaulting is an underground tunnel rather than the exposed surface one.

It eventually gets through and into the warren of 'inner' tunnels that aren't quite in the underground city proper yet.

There are sporadic explosions and gunshots for several hours.

Then there's a very big explosion that rather wrecks all the tunnels. The kind of explosion that makes one wonder if it was nuclear. The delvers seem to have been bracing and evacuating specifically for this.

The dragon is dead, after that. So are some delvers.

They're never in time to save everyone. That's true no matter when they arrive and sometimes it's true about people who die when they've already shown up.

They head back in to get an aftermath report from the delvers when things have settled out in there.

"What a regrettable series of events" is the overall attitude among the delvers.

The Security Chief says that they only had the one antimatter bomb. It was by his order specifically that it was used, after the dragon killed three security personnel and shrugged off everything they did to it. It was technically starship fuel.

He'll swear off having any responsibility or authority in a hypothetical delver member state, for having ordered its use, if that is necessary.

It's not strictly necessary but it might improve their press.

They could use some healing. And after the cleanup there are more meetings to be had to handle the details of joining, which proceed much more soberly and with much less dithering, now. The three delver communities are collectively writing this place off. They want a mountain or two somewhere not in this world and enough immortalities to cover everyone and will save up for resurrections beyond the initial set themselves.

- are they positive about the not in this world part. Separation from the local magic system could have side effects. They haven't noticed anything in living people traveling away from the world yet, but permanent settlements, or having children, might be another matter.

They're pretty set on it but could settle for space on a planet where no dragons are allowed, ever. On the whole planet.

That can be arranged! What do they think of this nice moon around this gas giant? They can have an arcology today and Terraforming can be done with the place in two months if they'll take an out of the box standard ecology. And how many immortalities are they going to need?

22,500 plus however many resurrections they get, for a healthy ready reserve going forward. You keep reserves of important things, they're treating that as obvious and unquestioned.

Can do. They can be toted to the moon as soon as they're packed.