A flameskull is particularly nasty creature whose dim recollection of its former life lends a certain air of pathos to its suffering. Permanently killing a flameskull so that its spirit can ascend to the next realm is not easy. You must splash some holy water on it after you break it apart, or it will reform.
The bleakborn is a fairly standard undead thing, smarter and tougher than a zombie, but still not the kind of undead the leaves an experienced zombie killer sleepless at night. This was was being used in service of what I assume were the Red Wizards of Thay. If there were more on the airship that transported him to us, the copper dragons graciously killed them all.
Bleakborn are drawn to fire, so should we encounter more, we can use the flames to draw them out. That should make Garos happy, as he loves shooting fireballs in the way that most young men his age like to masturbate.
Speaking of Garos, he was so impressed by the copper dragons attacking the airship that he included a drawing of one to go with the bleakborn and flameskull drawings. This book is to catalog the undead and collecting knowledge that threatens to be lost if not for undead scholars like myself. Dragons do not need me to catalog their form and behaviors, which are well-known. Still, it was a spiffy drawing and, more importantly, the more time Garos spends with his paints and paper, the less time he will spend accidentally bringing forth earth elementals. The bit where the dragon seems to be speaking to a gnome is pure invention on his part.