The Consecrated Ruin of Boccob is a cult that grew from Mother Whisper's growing influence over the material plane. Ultimately, her plans are to manifest a planar occlusion which would obliterate all life as we know it - but would condense every source of arcane energy into one central location...her
Now a greater undead, Mother Whisper was an acolyte of Boccob himself before she murdered his physical form and usurped him to assume the position of godhead of the arcane. She was quickly apprehended by his guards and submitted to tribunal. However, an execution would prove to be impossible. Before enacting her murderous plan, knowing she would likely be killed herself, she learned the secrets of soul separation - cleaving her spirit from her body and hiding it.
At the executioner's block, she laughed, taunting the guards and the crowd. Then the axe fell. However the blade didn't slice through flesh and bone as expected. Her form instead turned to ash around the blade, slicking like wet sand. Still laughing, she rose to her feet, the axe passing harmlessly through her body, sending flakes of ash blowing into the wind.
Horrified, the crowd hesitated to react - but in that moment, Boccob manifested in his astral form and pronounced Whisper's fate. She was held in a temporal stasis, quartered, and her pieces were scattered to the planes - with her head banished to Neth.
After her imprisonment, Boccob met with his guard and ascribed them with a divine duty to protect these pieces of Whisper's body from detection - as well as instructing them to dedicate themselves to finding her soul so that it may be destroyed. This small guard grew to be known as The Order of the Sight Unseen - the order that the party is familiar with today.
Unfortunately for humanity on the material plane - the last great war brought with it the discovery of the Order of the Sight Unseen, and after they refused to assist the Caerynian royal family - its destruction.
This caused a weakening in the ties that bind Wisper and she suddenly found herself able to reach out with her mind. She searched for what seemed like an age for someone vulnerable - yet powerful enough to influence - and that's when she met Yob - a common girl, but one with great ambition and latent power. Whisper named Yob her prophetess and through her founded the Consecrated Ruin of Boccob - an order dedicated to finding and reuniting Whisper's body so that she might return and reclaim her divine seat.
However, it should be noted that Whisper's true intent is more than just godhood - she wishes to cause an occlusion of the planes - forcing them all to exist in the same space, which would (theoretically) reveal the true source of all magic.