Starfinder: Remnants
As is the case with war, Scientists were hired to experiment and create new weapon systems to be used against their enemies.
Deep in a hidden facility, a group of researchers managed to unwittingly open a gateway to the negative energy plane, despite the safeguards in place to prevent such an accident.
The negative energy the scientists unleashed from that horrific plane instantly killed them and the entire staff. Thankfully the experiment was guided by an AI which managed to shut down the gateway, preventing many of the creatures which call it home from crossing over. By then the damage had been inflicted.
The negative energy released sapped the very life from all living creatures in the facility, aging them so rapidly they quickly turned to dust, but their souls remained. These silvery-white apparitions had the exact appearance as they did in life, and milled about the facility, not interacting with one another or seemingly were capable of any sort of coherent thought.
The AI in charge of the facility contacted the corporation responsible for the experiment. Not knowing what to expect, after the AI announced the strange energy had dissipated, the CEO of the company sent in a team to investigate. They were equipped standard gear, which proved to be all but useless in the end.
Upon breaching the facility, the team encountered their first Echo. The creature paid them no heed; it just floated past without any reaction. This was unsettling, but the team continued.
After encountering several dozen of these creatures, the team deemed them to be harmless, but were made uneasy and were constantly vigil due to the nature of the beings they encountered.
During the exploration of the facility, one of the men accidently cut himself and what happened next proved how wrong they were concerning the ghost-like remnants. There were three of the creatures inside the chamber and as soon as his blood hit the air, they turned black as the deepest depths of space and the eyes turned red. They swarmed upon the hapless man and drained his life-force reducing him to a pile of dust in mere seconds. Even his equipment and weapons were destroyed. Moments later an echo of what he used to be appeared above the dust.
Of course this caused the others to panic and attack, using their weapons. Soon every creature in the facility converged on the location. It was discovered they were able to harm or destroy the strange apparitions using energy weapons, and it was the only thing which saved them. Out of the thirty men and women who entered the facility, only three escaped with their lives. They discovered even a simple touch, intentional or not, caused horrific damage and left them scarred and changed forever.
Only energy and silver are able to affect these creatures in any manner.
Fortunately there are only three ways to attract the attention of these creatures. One is through blood. The other two methods are through violent combat, or through very loud sounds like explosions or shouting. Any of these conditions will automatically draw the creatures from anywhere within 250 feet, even through walls. The creatures are incorporeal and can pass through any barrier – except for force fields or energy shields.
If any of the three conditions are not met, it is quite possible to pass through massive mobs of these creatures. Just be careful not to accidentally touch one, or the person touching will suffer the damage and Constitution loss. This does not trigger the creatures attack, unless the victim screams (Will saving throw, DC 10 plus damage sustained), or the victim attacks the strange apparitions.
Anyone who is completely drained of their Constitution by these creatures will crumble into dust, all gear and items carried included, and will rise as one of the creatures in only 1d3 rounds. Once someone dies at the hands of these creatures, they are permanently dead and can only be recovered via a miracle or wish.
There is but one barrier which will prevent these creatures from passing. They cannot pass silver, so something as simple as a line made of silver will prevent them from moving past. It requires silver powder, which costs 1 gold piece to create a single length equal to 1 foot.