From the document:
Adventure Supplement for B1 In Search of the Unknown
The true secret behind Quasquenton did not become lost with vanishing of Zelligar and Rogahn!
This adventure supplement details several encounters to bolster TSR’s B1 In Search of the Unknown.
[F]or eight characters of first to second level using the First Edition game rules.
Free distribution via e-mail for existing customers, September 2015. Includes additional background, six encounter areas, one new spell, six new magic items, and two new monsters. Includes full-page map.