Slither since it just sort of randomly appeared in the middle of The Last Apprentice series with all new characters. Plus the name turned me off, I figured Slither would have something to do with a snake of some sort. Thankfully it did not.
Slither is what I would consider a creature of the dark since he consumes human blood and takes souls. Yet despite that I have not seen a mention of any of the creatures that appear in Slither in the previous books in The Last Apprentice series. Despite Slither being a monster who had seriously messed up ideals about what female humans are for I liked him. Sticking to a strong code of honor and going out of his way to keep bargains Slither was complex with many layers to him.
The three sisters were annoying most of the time. Nessa was interesting at times and while she constantly went out of her way to protect her sisters I never thought it was fake. Having been sold to Slither Nessa knew her life was over once she was sold in the slave market. I think that is what gave her the courage to keep doing all she could to protect her sisters.
The writing was well done and I hope that Slither was not just a filler book. If just Slither makes another appearance in the series I will be a bit disappointed because Nessa has a fairly good foundation as a character to make another appearance.