Iffy's knowledge of the PtD edition (many spoilers)

How to access:Save Dusk and talk to her, after that you need to head to the Archives where there is a blue golem that isn't there normally. Kill it for the broken pendent and (talk to Dusk and/or head to the antiquated set). There will be a ball of darkness that'll take you to the new content.(Placing the lordvessel is needed to get to the Archives)

The Sanctuary Guardian boss is at the beginning of the content, cutting the tail will give a poisonous whip. Killing will give you the Guardian soul.
There is a mushroom named Elizabeth, sells Dusk's spells and gold resin.
Guardian armor (the new stone) is scattered around the area.
The second boss is Artorias, killing him will give his soul and Domnhall will sell his weak but light armor. Returning to Arty's area, you will find Ciaran paying her respects. She will ask you for Artorias' soul. Saying yes will give you the Gold Tracer and the Dark Silver Tracer (DST puts the bandit knife's 135 crit bonus to shame with a 160 bonus.)No idea what happens if you refuse to give the soul or if you attack her. DTS has an S in dex bonuses and may be poisonous.
The Gold tracer off hand attack will change if the DST is in the right hand.
New pyro does pure physical damage, I think it's a fireball.
Merchant named Chester is useful for pvpers, selling Green blossoms, moss, Lloyd talismans, and arrows(feather included).Killing him will give you his clothes and Hat/mask.
The third (final) boss is Manus, Father of the Abyss and possibly the pygmy. Does massive damage, even on NG. Not much else to say except "Prepare for dark magic spam when he gets half health."
You can kill the Black dragon that appears.
There are supposedly walls that can disappear if Cast Light is used.
Hawkeye Gough is in the content somewhere, he supposedly helps kill the black dragon.