The Solemn Blades are a lesser known clan in Aetheria. Their leader is unknown, as the group only appeared briefly in WV15/8BW6. Their clan symbol is a red sword, which is painted on the breastplates of each member.
When Runt was coming back from a general store with milk to cure Ophelia‘s food poisoning, he came across three angry human members of the clan chasing an invisible thief who had stolen a ring from them. After encountering them and being mistakenly accused, Runt realized that the thief was his old rival, Cogboggle, invisible due to his hide ability. After he dashed off, the knights angrily continued to pursue him.
When Cog arrived at the Enchanted Dragon, obsidian ring in hand, the Solemn Blades presumably tracked him down, due to angry shouting being heard from outside the inn. After Brio angrily orders Cog to return the ring, the thief in training reluctantly does so, vanishing again to return it. Although the exact outcome is unclear, it is presumed that the Solemn Blades let him go, as he is perfectly fine later on in the book. Either that or he returned it to them whilst invisible.