What is the Great Filter?
The Great Filter is a barrier that hypothetically exists between terrestrial civilizations and interplanetary civilizations. There are in truth plenty of other potentials for the great barrier, but discussions about the emergence of microbial life, or the jump from single celled to multicellular organisms makes for boring reading, and little use to a science fiction story.
As technology develops, the ability to kill and destroy increases at a frightening rate. Emerging cultures must temper their violence as their power grows, otherwise they are wiped out by their own abilities. This is a common theme in apocalyptic lit, which often is the voice of reason against senseless and unlimited war, and the increasingly sophisticated and powerful weapons being created. A variation on this theme is the conflict between man and technology, the war with the machine trope.
The Resource Wars were a period of extreme violence, with almost every weapon built being used in some form or fashion. The number of casualties inflicted by nuclear, biological, chemical, and electronic weapons cannot be truly known due to the amount of damage done.
We Require More Minerals
The march of civilization requires greater and and greater resources, both in materials and energy. A civilization can exhaust it's natural resources and either become stagnant with a much decreased level of activity or collapse completely. Examples of this can be seen on Easter Island, where the island civilization was greatly reduced after the islanders destroyed their supply of trees, leaving their island barren and vulnerable to erosion. This is also a visible issue in competitive RTS games where matches are sometimes determined not by units or tactics, but by who runs out of resources first.
The Resource Wars were set off by rapidly dwindling supplies of fossil fuels. Investigations into alternative fuel sources proved too little, too late.
The Universe is an innately dangerous place, filled with asteroids, gamma ray bursts, and coronal mass ejections. Life on any planet is at the mercy of the hazards of space. History is filled with regular extinction level events and craters miles wide. Hollywood has made a genre out of this potential filter with films like 2012, Deep Impact, Armageddon, and so forth.
The Cosmic Era has yet to face the challenge of a random planetary extinction level impact or event.
There are other aliens in the universe, and they are not friendly, not interested in making friends, and generally make sure that they do not have any competition. Others are simply megapredators that live on a larger scale than humanity. Examples of Predation include the Zerg swarm from Starcraft, the Tyranids from Warhammer 40k, or the Borg from Star Trek. Meeting these species is a death warrant for anything not their equal or superior.
The Earth has felt touches of predation, in the form of star whales, the larvae of the outer gods, but these have been very minor incursions. Humanity has lacked the technology or understanding to realize what has happened.
The greatest things can often be laid low by the smallest. Microorganisms can decimate, or even destroy a civilization, as demonstrated by the destruction of MesoAmerican and South American Indian tribes who were annihilated by smallpox, or the Black Plague, decimating Europe. The reverse of this appears on occasion in lit and film, such as the well known weakness of the Martians in War of the Worlds, common terrestrial bacteria.
The Cosmic Era has had it's share of epidemics, but the era will not end with a sniffle or a sneeze.
The ouroboros is the serpent consuming its own tail. An emerging civilization runs the danger of consuming itself, literally in the case of exhausting it's resources and destroying it's own environment, or it runs the danger of turning inward on itself. This has happened in the past, the closing of China. The nation remained isolated from the rest of the world, and stagnating technologically and culturally.
The siren song of technology is powerful, and the urge to explore and grow diminishes with the ease of entertainment and self distraction. There are arcologies in some cities that have functionally closed themselves off, finding greater interest in their own internal affairs, or the endless offerings of streaming entertainment. The technological navel gazing has made these towers into intellectual and creative cul-de-sacs and dead ends.
Things Mortals were not Meant to Know
There are hundreds of religions that all have spoke of the end of days and giant monsters, and and more. One by one, these have been unraveled, doomsdays have passed, apocalyptic events foretold have not happened, and no giant monsters have appeared to annihilate the wicked and the heretics. Toho gave us Godzilla, Lovecraft gave us Cthulhu, and comics gave us things like Galactus, Unicron, and other world devouring horrors.
The crux of arcanotechnology is That Which Man was not Meant to Know, and the use of the tainted technology has given rise to the appearance of Dimensional Fatigue Events, giant monsters, and unknowable intelligences.