My draft for a scenario that tries to be as grimdark as possible.
Life is an interesting thing, unique in the universe. A series of chemical processes conducted in such a way that it serves as a conduit for élan vital, a form of "energy" which exists beyond the physical universe. In tiny quantities and in simple systems, élan vital has no effect, but when a critical density is exposed to a very specific set of electrochemical reactions, it confers upon the living thing the gift of consciousness.
Élan vital is special in that it is not of the material universe. It is native to another universe "adjacent" to physical reality, an alternate dimension called the Etherium. Élan vital can only exist in conjunction with the particular chemical systems that exist in all living things; indeed, life is defined by the presence of élan vital within a chemical process. When these chemical systems fail, élan vital becomes unstable and is punted back into the realm from which it came; this process is more commonly known as death. Every return damages the local spacetime barriers in which the return occurred. A single return in one place at one time is minimal, several in a short amour time will weaken the veil between the worlds enough to let some élan vital consciousnesses to manifest themselves temporarily and in short bursts.
Several million in a short time will weaken the barrier permanently.
1918. The world was plunged into darkness by every horror mankind could suffer. The most overt horrors were those created by Man himself. The Great War tore Europe asunder, the Russian Empire fell to revolution, China was wracked with civil strife and an invasion from Japan, the armies of the United States and Mexico clashed in the frontiers of the West and South America’s dreadnought race became a full-blown war. The wonders of technology, from the diesel engine and the machine gun, to poison gas and the mechanical cogitator, made death easier to deal on the battlefield. But the true killers were inhuman in nature. The Spanish Influenza brought countless millions to their graves, particularly the young and healthy. The famines caused by war-ravaged supply lines and the poisoning of soil, water and air by the great powers’ conflict killed and weakened those that the Spanish Flu did not kill. By the middle of the year, a hundred million people had perished. The fuel for a total barrier collapse was ready. All that was needed was a spark.