The Dark Below is on its way to Destiny on December 9, and in the run up to its launch, the timed PlayStation-exclusive content (until "at least Autumn 2015") has been detailed by Sony in the latest PS Blog.
In the blog they detail the Strike mission, The Undying Mind ("You can step back into the coveted dangers of The Black Garden to face a timeless Vex machine-beast that has survived for ages. Prepare for firefights amidst vine-laden pillars and decaying paths in an effort to destroy the centuries-old evil"), and The 4th Horseman shotgun ("Four barrels, five shots, and a sizzlingly fast fire rate will eat away at the Vex legions in short order"). Check out the attached images for a more visual description. For more on the incoming expansion, head here.