Frankly, Svaltimas, even his living spirit, was far from weak, he was an enormous frost dragon, and this version certainly had the memories of everything up until the moment of his death, meaning that he remembered fighting Gravelord Dracula, even if it was more akin to an execution than a fight, any amount of experience fighting a Gravelord was worth a lot.
With his massive size, ability to fly with surprising agility, and powerful frost attacks, not just any undead could go up against something like that.
Nowadays, in the Undead Empire though, there was no need for a powerhouse to be around for such dire threats to be dealt with, fortified structures equipped with incredibly destructive weapons were everywhere, and these weapons could be used by any undeads, so even if a dozen of dragons as strong as Svaltimas attempted to storm into the Empire, they would be shot down before even getting through the border.
