Great One (aka. Black Mass) is a cosmic entity that confirmed as true main antagonist of tie-in story called Truth Behind The Salem Witch Trial, where it was revealed that He was the mastermind behind the Salem Witch Trials.
After centuries passed, Great One began to prepare His return to the world, where His influence was felt by Dr. Griggs and Sam Twoflower the snow fairy during the said infamous Witch Trials, as He sought for latest girl that worthy to become the Priestess of Ancient Religion. Great One also responsible for the creation of Morganians as well.
From what mentioned in Raido's assignments about lost civilazation called Lemuria that recorded in Augustus Le Plongeon's works, Great One was responsible for bringing Lemuria to it's glory. The same influence also spread through the following civilazation that rise after Lemuria, Atlantis, that appeared at the same moment where Lemuria in it's golden age. Both ancient civilazations worshipped Great One as their God. Great One also introduced them a power to manipulate matter and reality with one's mind alone, which later known as Magic.
However, after learned about The Great Old Ones' true nature and also their weaknesses, both civilazation began a revolution where they uses weapon that can banished, if not truly destroy all Great Old Ones as the cost of their great achievements.
The Great One is presumably amoral entity that too alien for humanity to understand as much as it's nature, just like majorities of entities within H.P. Lovecraft works, or at least that was it seemed, given that it's influence are what inspired Lovecraft for his works.
Powers and Abilities
Great One' powers are remain mystery, but implied to be God-like. It was capable to create the extension of itself that referred as Creatures of Darkness, where the most famous of these beast is Black Hydra that greatly resembles Hydra.
It's nature also too great for mortals to bear without affecting their sanity, though not apply for those with blood of magical creatures that runs through their veins like the snow fairy Sam Twoflower.