The Fun Bunch accidentally discovered the Underhouse without ever having heard of it. They accompanied
Tjeli in search of the remains of Valdas Manor, which had reputedly vanished decades earlier, presumed swept away by one of the violent possibly-magical storms that hit the eastern side of
Malfa peninsula in winter. In fact, it turned out that Valdas Manor was still there, but only sometimes; it had become the Underhouse, and was magically confused inside as out.
Valdas Manor is occupied by a banshee called the Librarian, whom nobody seems to bother much; an imp calling himself the Clerk, who runs the Library as a trade scheme; a bunch of darklings, who respect the Librarian and serve her interests in a rather haphazard way; and some orcs, who work for the Clerk. The chief of the darklings strongly suspects that the Clerk is in business for himself, and thus disrespecting the Librarian. There's a lot of politics here that is compeletely opaque to outsiders.
Tjeli seemed to either recognise the Librarian, or suspect who she was.
Valdas Nykomin, the occasional employer of both Tjeli and the Fun Bunch, later confirmed that the Librarian sounds like an ex-Maveren named Mera, Nykomin's half-sister by blood and a member of
Erling House.