At the end of Landy's last Skulduggery book, The Faceless Ones, the detective was sealed in the Faceless Ones' alternative world. Valkyrie is determined to rescue him, and she does after surviving another attempt by the Sanctuary, the organization that supervises the use of magic, to arrest her. Other factions besides the Sanctuary seek to kill Valkyrie. Skulduggery, too rattled by being tortured by the Faceless ones to pick up all the clues, puts Valkyrie and their friends in needless danger.
Dark Days starts out with various villains out to kill Valkyrie Cain, not as collateral damage, but as the focus of their murder plots. The ending of the first paragraph clues you in. Dreylan Scarab, the primary villain in this book, had two interests walks and murder. During his imprisonment for assassination he "... kind of lost interest in walks. His passion for murder, however, burned brighter than ever." The days are indeed dark.
The next chapter is the attempted murder of Valkyrie in her Stephanie persona ... but she is saved by a necromancer. I don't think I even knew what a necromancer was when I was twelve even though I read every issue of the comic Tales from the Crypt I could get my hands on. I even hid the comics under the stack of Disney comics so my mother wouldn't see them. All this by way of an illustration that kids have been reading "dark" subject matter for generations ... in spite of bans by parental or others. Bans don't work.
Landy uses the necromancer as a means to an end -- saving Skulduggery from an alternate universe where he was lost in the last book. In reaching that goal, Valkyrie makes decisions that the adults around her object to ... but she uses her own judgement to amass all the magical power she can gather. Valkyrie is growing up. Even though she takes direction from Skulduggery, she begins making her own decisions before adults can tell her what to do.
This fourth volume is fast moving ... with many of the surviving villains of previous books not always behaving as you might expect. Also, Landy has come up with some innovative magical devices to cause the maximum about of destruction if our heroes can't save the day.
One thing lacking in this book, though, was the humor. I missed it. Still, I'm champing at the bit to read the next book to see how Skulduggery and Valkyrie avoid her doomsday future -- which was well planted in the course of escaping all the plots against the duo.
Pence is a former cop and his home page has other information that could prove useful. Even before I had settled into a dark story streak, I had thought about how the average civilian might encounter a "street disaster" or "home invasion". Pence's info might help you make such a situation more real. -- Thanks to Jordan Ellender for the tweet that alerted me.