Another common theme is the supernatural. Many second riddles in horror stories are related to ghosts, demons, or otherworldly forces. The riddle might be a way to summon or understand these entities. For instance, a riddle could be about how to find a hidden portal to a haunted realm.
Isolation is also a theme. The riddles may lead the characters in the story to isolated places, like an abandoned asylum or a lonely cabin in the woods. These places are often filled with horror, and the riddle is the trigger that sends the character into that terrifying isolation. For example, a riddle might give directions to an old, forgotten place where something evil lurks.
Yes, death is a common theme. Riddles often lead to revelations about death, either past murders or impending doom. For example, a riddle could be about a place where people have died mysteriously, and solving it uncovers more about those deaths.