"The operatives encounter a group of Greys in an open field. Before the operatives get a chance to react, one of the Greys points a slick metallic pistol-shaped object at the sky above. The tool makes a sudden scraping noise and a strange twinkling mote of light soars up into the clouds above the operatives' heads. The operatives hear a deep rumble and then orange rain begins to fall."
Possibilities: (You could even colour-code the rain for different effects.)
a) the rain is a very strong hallucinogen, the investigators are soon flat on the ground having some very bad trips. Suggestions for hallucinations: they see it rain airplane fuselage and human body parts or more surreally, they see it rain disemboweled cats and disemboweled dogs
b) the rain is a strong sleep-inducing drug
c) the rain is human blood, pure and simple, the Greys use it to distract and intimidate; Sanity rolls everyone.
d) the rain is acidic, flammable, toxic, protomatter, etc. (but I like this artifact because it can be disturbing, while non-lethal)
The Mi-Go sometimes equip their little puppet-minions with cloud-seed flares so that they can deal with unexpected encounters of the human kind. The flares themselves are based on some highly advanced and unstable synthesized biochemical catalysts.
This is from the Ice Cave.