Fane of the Sea God

From the introduction:

Just as the Old Man’s Tunnel and the Devil’s Maw are local alternative entrances into the Dragon’s Delve, there are others not so close. Miles from the Fallen Duchy, along a distant storm-wracked coast, lies a portal that provides a way into and out of the lower levels of the great dungeon. This connection, via an ancient temple to a forgotten sea god, is all but unknown. It may, through rumors of its portal, bring adventurers seeking the Dragon’s Delve--or it might be a trap for the unwary, drawing unwitting explorers of the temple into an underground labyrinth much vaster (and perhaps more deadly) than they had anticipated.