I am ruling that Kas and Vecna broke off the confrontation.
Neither of them was killed.
Hundreds of thousands of Kas's people, and hundreds of thousands of Vecna's people, now lay dead on the fields of what used to be Nyrond.
(This happens because someone e-mailed me that it did.)
The spelljamming dwarves arrive over the Battle of the Flinty Hills with their mountain spelljammers.
However, one of those mountain spelljammers is empty.
And being used in a suicide attack.
It goes crashing down onto the battlefield, and produces an explosion of titantic proportions.
The light is seen across the entire eastern Flanaess, and it is heard in Rauxes, a thousand miles away.
Unfortunately, although the dwarven intention was good, their aim was not, and mountains do not make precision guided missiles.
Several hundred thousand beings perish instantly.
The God Emperor was saved by his Seers, who teleported him out just in time.
The rest of his generals are killed.
A third of the entire army of the Dark Union is instantly killed.
Half the Lost Elves are killed.
However, the Mountain was aimed at the battlefield, and the battlefield WAS the Flinty Hills.
Women and children scream their final screams as the underground cities of the Flinty Hills all collapse at once.
The civilization of the Flinty Hills is obliterated.
The Host of the Flinty Hills dies with their people.
The Host of Garrel Enkdal is wiped out.
The Nyrondese lose half their force.
The Gnomish Spelljamming Fleet loses a quarter of it's ships.
The Sky-Sea League loses a quarter of their force.
The Seldanoran Host loses a third of it's force.
The Swanmays lose half of their people.
Half of the Giants die.
A third of all of the rest of the forces involved are killed.
There is now a giant crater, a mile wide, where the center of the battlefield used to be.
In Nyrond to the west, the battle rages, lit up by the red glare that now fills the sky to the northeast.
The forces of Kas have wisely retreated into Nyrond's cities, and they are holding out against the Legions of Vecna, making them pay for every inch of ground gained.
It is a terrible battle, without quarter or mercy, or even sanity, as the two armies of fanatics and undead battle it out.