The mythos is simultaneously a pantheon and a bleak existential approach to reality--one of the most certain and most dangerous paths to power in the universe. For more information on the source material, check out the wikipedia entry on H. P. Lovecraft.
The Mythos in Suburban Senshi
The Mythos' first contact with the Suburban Senshi occurred when Jedite was banished by the Cindy S.M.A.R.T. doll to the realm of the outer gods (Suburban Senshi episode 9 and 10). Where this would have been the end of lesser creatures, Jedite killed and ate Cthulhu, then hollowed the carcass out to make a sort of suit. His subsequent attempts to drive the remaining Great Old Ones into a formidable fighting force bore little fruit, and he gladly took the chance to escape when it arose.
The Mythos is not well-understood by most visitors to Ten'ou House. Only a few studied magic-users, including Yaijinden, Raihosha, and The Traitor are fluent in its use. Others have been traumatized by it. Others still are actually parts of it, including the book-loli grimoires such as the Necronomicon, the Seven Cryptical Books of Hsan, and The King in Yellow.
The Mythos in the Extended Universe
About the Mythos: Summary
Works written by those experienced in the Mythos describe an existential universe devoid of greater meaning or purpose. Humanity owes its existence to chance and accident, and is liable to be obliterated by a similar chance or accident. While there are friendly gods and spirits that are interested in human life, the majority of Mythos powers are blind to and ignorant of human interests the same way humans are ignorant of individual ants. Forces of the Mythos are as dangerous as any ancient spirit; the lost world of Carcosa is a fabled story of loss when one world blindly played with forces beyond its ken. While in the modern day most such powers are trapped within The Void or the Interstice, many can be freed from their prison...at least for a short while.
No easy taxonomy of Mythos creatures exists because few individual races bear any relation to one another. A folk taxonomy from the perspective of the humans of Earth classifies Mythos-affiliated beingsinto four general categories: Outer Gods' (mindless cosmic forces such as Azathoth, Shub-Niggurath, and others), 'Great Old Ones' (singular entities of incredible power including Cthulhu and Hastur), 'Servitors' (races of creatures that mindlessly serve more powerful beings), and 'Independents' (less-powerful entities that tend to act of their own volition, such as shoggoths or mi-go). Servitors and Independents are often sub-classified into Greater and Lesser, depending on how likely it is for a well-armed (but otherwise-mundane human) to defeat one.
The forces that drive Mythos powers are closer to hyperdimensional science than to qi or mana, meaning that anyone who possesses a formula for summoning byakhee can do so under the right circumstances. However, performing Mythos 'magic' is insidiously dangerous. Rituals performed incorrectly can malfunction catastrophically (with visible or invisible effects). Spells performed correctly can still twist the caster's mind or body, as the human psyche bends in ways it was not designed to bend--and the mind never entirely bends back. Rituals penned in the books of cults often encourage worship or deference to a particular entity, so the wise prospective independent Mythos sorcerer is extremely careful what they take from books they find. Few spells are cast strictly from memory, and most sorcerers work out of a grimoire for easy reference--all the better if their chosen grimoire is sapient and capable of guiding them through the rite itself.
The Mythos and Earth
While Mythos creatures pose an undeniable physical threat to humanity, much of the danger that comes from fighting these creatures is psychological. Average Human minds cannot process the data that perceiving Mythos threats forces into their consciousness, and exposure to horrors beyond knowing can trigger psychotic breaks from reality. Extended exposure tends to make such breaks permanent, or spurs even more dangerous behavior; government agencies often keep personnel tasked with tracking or eliminating these forces under constant psychological review.
A multitude of Mythos cults exist on Earth, dedicated to individual godlike entities or toward study of the arcane principles of the Mythos. Some societies work partly in the public eye, passing as a social club or charity organization. Others are too small, or too isolated and entrenched in the local culture, to require much secrecy. Regardless of their size or origin, known Mythos cults are considered a blight on society by the wise and exterminated when possible by independents or government organizations.
The chief difference between cults dedicated to Mythos powers and cults dedicated towards other dark gods is that Mythos powers are, for all their power, seldom aware of their would-be servitors. Cults that claim to receive power from their chosen object of worship are often delusional; their sacrifices, adulations, and sacred rites would call up horrors and monsters whether or not their chosen god existed or not.
For most of recorded human history, knowledge about Mythos powers has remained secret and concealed. Monster-hunters kept just enough knowledge to cast threats down while other cults and independents conspired with inhuman races or with one another to gather it. Spikes in Mythos activity in the early 20th century sparked a wave of modern interest (and a concurrent crackdown by organizations and governments interested in suppressing this dangerous information). All in all, humanity has come a long way in knowing its enemy. Dealing with Mythos cults and eruptions of Independent and Servitor-class threats is an established purview of many paranormal organizations, including UNIT, AEGIS, Torchwood, as well as independent monster-hunters. The awakening of a Great Old One or the summoning of an Outer God would be an apocalyptic event on the scale of the Cretaceous meteor or worse, and most organizations freely share information (sometimes without the permission of their superiors) to prevent such threats from emerging.