Hey Storytellers,
After reading the DnD Monster Manual Version 3.5 there is a throw away line for a shape shifting spider.
This spider has three forms (like a lycanthrope or werewolf): Medium sized spider, medium sized hybrid humanoid, medium or small sized humanoid. They can stay in any form indefinitely and they take the same form each time.
Listen to us create a quick scene that you could play using an Aranea as a small female halfling that runs a tavern or brothel and stalks her victims based on their patronage in her establishment whenever she might get low on money.
Her victims are powerful and wealthy in this town so they won't report these kidnappings because they don't want to appear weak. Then the players are hired by the latest kidnapped person's family.
See how we give you different options to give information for your players to figure out who the kidnapper is and even four different possible endings that you can plan for but your players will ultimately choose (5e stats link in our resources page).