Rick Lai at the Lovecraft eZine has put together one of the best writeups on the Dark Shadows "Leviathans" storyline that we've ever read. Be warned, this is a pretty in-depth look, and isn't something to casually glance over. At a touch over 14 pages of text, this covers all of the storyline, including the appearance of the Leviathans in the licensed storytelling of Big Finish productions.
Trust us, you REALLY don't want to miss this.
The Collinsport Horror: Dark Shadows and the Cthulhu Mythos