This book had more of an emotional impact than the previous books so while the story itself was kind of meh for me, it did hit me right in the feels and that's telling for good writing and characterizations. Seraph's story is heartbreaking and veers a little off the previous books' formula.

So Seraph is the creature Collector Mary was basically keeping in a closet drugged up, only dragged out by Gloam to fight other monsters for Mary and her military buddies' entertainment. During Rig and Gloam's escape from her compound, they release all of the collection except Seraph but are too tender-hearted to also just leave him there. They loaded up his cage and dragged him back to the base camp only to leave him outside the walls for safety. Gloam is determined to find out more about Seraph and to try and communicate with him but, after years of literal torture and no interaction, Seraph is basically feral.
Lilac, the camp's designated assassin, seems to immediately lock onto Seraph (much like the past humans and their monsters) despite Serpah's grotesque appearance. If you didn't know, a seraph is the Biblical angel with eyes all over them and Seraph here is described as having multiple eyes all over his face, seemingly independent of each other and seemingly of different species of monster entirely. He's described more or less as Venom (Spider Man) but with a lot of eyes. His origin story is even not that different.

Seconded only by Wyn and Danny, this might be the most unbelievable coupling. Like, we've already known Lilac is a bit of a weird wildcard but the instant attraction here is bizarre, albeit coming from my entirely judgemental standpoint. The sex starts too soon and Lilac is too trusting. It's a wonder if he'd be attracted to any super dangerous thing that's slightly humanoid with the other "monsters" he's met being to tame for his tastes. His childhood as we're told does not indicate the kind of trauma you'd expect for someone who looks at this caged nightmare and gets soft eyes, you know?
Despite this, it does end up being a sweet story. Lilac and Seraph's personalities blossom together and we learn they're both not just single-minded killers, though they can be when needed.