Quest graph.

The Librarian overheard some illagers discussing a project they were working on in the old ruins where the Arch-Illager once used to conduct experiments. After translating some old runes in the library towers, you (rather foolishly) read the runes out loud and awoke a Sculk Monstrosity that must have been slumbering underground for a very long time. At least now the villagers of Frozen Highlands won't have to worry that the illagers will wake the monstrosity when you're not here to stop it.