Epic Comics is Marvel's on-again, off-again imprint that was created by legendary Marvel E-i-C Jim Shooter in 1982. Over the years, it has been put to various purposes but in it's original form it was used by Marvel to publish creator-owned and adult-oriented material. Marvel's precursor to DC's Vertigo imprint.

From 1987 to 1990, Epic produced a line of adult-oriented superhero comics that all took place in a shared universe, not unlike the New Universe which Marvel was concurrently publishing. Unlike the New Universe, the Shadowline Saga was a boutique operation -- the comics were produced bi-monthly in an upscale "graphic novel" format featuring painted covers and excellent paper stock.

They also differed from the New Universe in terms of their content. All three Shadowline Saga books were written by Dan "D. G." Chichester and Margaret Clark (with occasional help from Chuck Dixon) from concepts created by comics legend and then-Epic editor Archie Goodwin. As a result, the Shadowline Saga had a very tight continuity. Artistically, the Shadowline books stood out as well. Artists Kevin O'Neill, Bill Sienkiewicz, Gray Morrow, John Ridgeway, Denys Cowan, Dan Spiegle and Klaus Janson eastablished a stylish and progressive look for the books that set them apart from contemporary imprints and obviously influenced the occasional Shadowline work of younger artists like Mark Texeira, John Romita Jr., Mike Mignola, Jim Lee and Mike Manley.

The end of the Shadowline Saga came simply due to poor sales but unlike most of the dead universes, the end was not abrupt. The Shadowline creators were allowed to finish their story in a new book featuring all of the Shadowline characters. While the overall storytelling was admittedly uneven, the Shadowline books at their best were some of the best comics available at the time, period, and were in many ways ahead of their time.

Some reference links:

Wikipedia article on the Shadowline Saga.
Interview with Dan Chichester.