Primordial - D

Drain = The Drain is what happens when mortal minds attempt to mix with the vast, alien minds of the Primordials. The unfortunate mortal may be drained of their sanity, or worse - transformed into a lifeless husk to be used as a plaything by the Primordial in question.
"De Vermis Mysteriis" = Or Mysteries of the Worm, this eldritch grimoire is common to members of the Order of Gnostics. For more info see this Link.
Deep Ones = The Deep Ones are a race (or races) of lesser Primordials who are mortal and worship the larger Primordials as odds. They live at the bottom of the world oceans where they build cities for themselves deep within maritime trenches. They are also at war with the Mermaids (so Corsairs or Paladin Adepts won't like the Deep Ones either). See Link for more. 
Doom That Came To Sarnath = A story of the destruction of a city by some of the lesser Primordials. See Link for the story.