Phantom (Final Fantasy)

The Phantoms are a now extinct extraterrestrial species, the spirits of which invaded Earth on the Leonid Meteor. Not much of this species is known, but what is known is that they were a biomechanical race.

Overview
The appropriately named "Phantoms" were invisible to the naked eye. When the Leonid Meteor fell on Earth and sudden worldwide deaths occurred, the blame was placed on a new type of virus. Many living things suddenly died across the world — thousands of plants and animal species — disrupting the natural cycles.

Dr. Sid and his team of bio-etheric researchers were the ones who revealed the existence of Phantoms, and thanks to the advancement of bio-etheric technology six years later, the United States Military Forces (USMF) began its campaign against the Phantom invasion. Bio-etheric weapons were developed, and barrier cities were erected as defense against the invasion.

The Phantoms were a hostile race, and attacked any living thing in sight. This also included the larger ones — those resembling beasts — that can be found in the wastelands such as in Tucson. Their bodies can go through any medium — like ghosts through a wall — and if this medium happened to be a living being, the Phantom would extract the being's soul and consume it.

Dr. Sid's research discovered that the number of Phantoms remain constant; that a Phantom will reappear when killed. He also discovered that there seem to be no relationship between the "soldier" Phantoms and the beastly Phantoms that roam the wastelands. It was then concluded that the Leonid Meteor was no alien transport as they had assumed, and that the Phantom invasion was not an invasion at all.

History
A discovery was made by Dr. Sid on bio-etheric energy that exist in all living things. Phantoms contain a unique wave pattern, and using this information, Dr. Sid created a bio-etheric barrier that can repel Phantoms. This technology was used to create barrier cities around the world which protected the human population on Earth from the alien invasion. Phantoms are left to wander the entire Earth.

Eventually, Dr. Sid's protege Aki Ross discovered from her dreams after being infected by a Phantom, that the Phantoms were actually spirits or ghosts of aliens from another planet. A cataclysmic weapon unleashed in the midst of the civil war on the alien planet wiped out its entire population. The aliens carried on their hatred even until death, which explains why most Phantoms are hostile. The meteorite from which they arrive is actually a chunk from the alien planet, which carried with it the alien ghosts and the ghost of the alien "Gaia". At the end, the spirits Aki found and the Phantoms cancel out each other, erasing all existence of Phantoms from the planet.

Biology
Phantom cells can infect bodies and souls, the latter of which they can also eat. In their current spirit form, they cannot be destroyed, but they can be purified. A Phantom Ghost is invisible but can turn itself visible with the radiation of a Barrier City.