The storied mansion in Arkham's French Hill district plays host to all sorts of parties these days. It isn't hard for you to get inside, and after mingling briefly you learn that Lina Husserl was a frequent attendee, though most of the guests express relief that she has been absent lately. Your questions about her behavior are met with stories of her obsessing over a locked door on the top floor of the manor.
You make up an excuse to leave the conversation, and visit the door yourself. It stands like a slab of cold, black stone, with a symbol etched into its surface: three arrows, fanning apart as they point up towards the ceiling. You reach out to grasp the door's handle, and are shocked to find it unlocked.