This universe has a civilization of humans! And some other things. It's not crowded in the way Edda is but it's not just the one planet either. The humans might be easiest to start with. They're on that planet over there, it's not Earth but it has very Earthlike conditions and bronze-age humans living in cities and villages dotted across two large continents. Depending on how thoroughly any prospective visitors look they might find other things before visiting.
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.
"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.
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.
They're pretty set on it but could settle for space on a planet where no dragons are allowed, ever. On the whole planet.
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.