One of the things that FS tackles is the Annunaki (the ancient ones who built the moon-sized jumpgates at the edges of the solar systems of the universe), the existence of ancient evils and intelligences in the dark between the stars, and the Holy Flame of the Pancreator as revealed to his prophet Zebulon and his apostles and disciples.
Aside from the echoes of various monotheistic religions found in the Orthodox Church, there are also various heretical beliefs and proscribed knowledge such as Antinomy -- a label for certain worships and forbidden knowledge that are 'against the Name of the Creator of All'.
And into that space, we find the need for the aforementioned ancient evils and intelligences.
Trail of Cthulhu has that in spades. But what makes this particular 'monster's manual' different from other listings of the Cthulhu pantheon and creatures is the work that Kenneth Hite has put together into creating alternative options for each of the well-known and less-well known elder Gods and Old Ones from the pantheon.
As the text itself says: "This book takes a similar approach to these beings, providing as many contradictory explanations and alternate versions for the Mythos heavyweights as possible."
Here are some examples with some staples of the pantheon with their variant definitions:
Original -- Azathoth, the blind idiot god, exists at the center of the universe, dwelling beyond normal space-time. Its amorphous form writhes eternally to the piping of demonic flautists, attended by mindlessly dancing lesser gods. Azathoth is the ruler of the Outer Gods, little worshipped on Earth, as it provides not even attention to its would be cultists. If summoned, it blasts the area around it, leaving cracked boulders, pools of alkaline water, and dead, splintered trees.
Variant -- Azathoth is an emergent intellect – a titan – created in the immense pressure at the event horizon of the supermassive black hole at the center of the Galaxy (in Sagittarius). The “hellish piping” is the howl of high frequency radiation emitted from its “prison,” the black hole’s Schwarzchild radius. Its sentience exists in hyperspace and maintains instantaneousdipole communication with other black hole entities, its “attendants.” The Tunguska blast of 1908 was an attempt to summon Azathoth to Earth.
Variant -- Azathoth was the leader of a revolt against the Elder Gods who impose order on the universe, without which matter itself could not exist. He failed and was hurled back to the endless wormhole moment at the beginning of time.
Original -- The Great Old One Cthulhu dwells in the sunken basalt necropolis of R’lyeh, miles deep at the bottom of the South Pacific. He sleeps eternally while there, sending horrifying dreams to mortal men, tipping some into madness and others into his fanatical worship. Someday R’lyeh will rise again and Cthulhu will wake, freed once more to raven and slay, freed to rule the world.
Variant -- Cthulhu is an Outer God, the incarnation of (or a sentient facet of) gravity, one of the four fundamental forces within our space-time.
Variant -- Cthulhu is an infra-dimensional entity that has only a conceptual existence within the human “R-complex,” the brain stem and limbic system left over from our primordial reptilian ancestors. This is why he appears only in dreams, high-stress encounters (such as shipwrecks), and artistic impulses. He is attempting to create a critical mass of believers so that he may “emerge from R’lyeh” and open the eyes of all.
Now, some purists may take issue with these reinterpretations, but they do provide the FS Gamemasters ideas for blasphemous creatures for use in the Fading Suns context. All you need to do is change the names!