Yuglid

The Yuglid are a slightly humanoid, insectoid race. They have armored bodies covered in plates reminiscent of crab shells, long necks, face tentacled, beady black eyes, taloned hands and feet, black blood, jagged teeth arranged in an underbite, and wings that can fold up. They are faster, stronger, and much longer-lived than humans, living for eons. They also have the ability to grow human skin coverings that allow them to blend in with the human population, though the growing process takes months. Some of them are also capable of very powerful glassdancing sorcery.   Several characters in the story are revealed to be Yuglid, including the glassdancer assassin who killed General Stavri and his officers, The Tall Man, Aristanes, and Professor Sumala Volos. Some of these also have their own monikers, denoting their personalities and functions in Yuglid society. For example, the Tall Man is The Angry One, Aristanes is The Schemer, and Professor Volos is The Thinking One.
STORY
The Yuglid and their ways are mentioned in a variety of instances.
When talking to Kissandra Vorcien, Madame-under-Magna casually mentions "things that walk in the night" that she has seen, but insists it would cost a king's ransom for her to talk about it. Kizzie does not understand the reference, ignoring it as fanciful exaggeration.
Squeaks reports seeing a "monster" in the woods, but no one really believes her, dismissing it as nerves combined with low morale. Later, Squeaks feels the sorcery of a very powerful glassdancer and reports it to Idrian Sepulki, who thinks it might be an assassin. He climbs onto the roof to investigate only to realize that the creature is not human when he sees it up close. The Yuglid does manage to escape, and the resulting rumors suggest that it might be a monster Devia Kerite brought with her from abroad.
The Tall Man, known amongst the Yuglid as The Angry One, follows Kizzie's investigation, making sure she does not meddle in the affairs of the Glass Knife. She sees him multiple times, including immediately after Glissandi Magna's suicide, suggesting that Glissandi saw him and realized dying was a better alternative to giving up her secrets about the Glass Knife and the Yuglid.
At the Zorlian Mansion, Baby Montego and the Tall Man fight to the death. After believing he defeated Baby, the Tall Man tells Kizzie that the Yuglid will remember him for his fighting ability and sing songs about him long after humanity is destroyed. When Kizzie asks him if there are more of his kind, he responds by saying that she has no idea.
After the slaughter at Zorlian Mansion, Professor Sumala Volos (The Thinking One) and Aristanes (The Schemer) briefly discuss their situation. According to Aristanes, humanity usurped the world from the Yuglid, and he means to take it back from them. The Thinking One disagrees, preferring to assimilate and study amongst the humans. She insists on taking the heavily wounded Kizzie and Baby with her. The Schemer points out that they would learn of her true identity as a Yuglid, but she responds that that they have revealed themselves to humans a few times before. When The Schemer threatens to kill her instead, she simply says that her sister would come looking for her, setting back his plan for centuries. The Schemer then leaves, ignoring the Angry One's corpse on the floor, which the Thinking One notes as a tiny insult in Yuglid culture.