Imaginary Warhammer

There's a story somewhere where the Tau are fighting against space marines. The Tau are a much younger race which is just barely learning about the outside galaxy. They think they are the height of technological advancement and that colonizing the galaxy will be a cakewalk. When fighting the marines, they think that the relatively small detachment of marines is the entirety of humanity.

At one point the Tau manage to take down a marine dreadnought, and a Tau soldier goes up to it, thinking it's just a big war robot. He gets weird readings about organic matter and manages to get the dread open to find the desiccated, frothing marine inside cursing him out while he dies. The Tau scans the marine and realizes that the marine is older than when the Tau race had discovered how to make fire. That was a moment of existential dread, realizing they were fighting an ancient race that they didn't understand at all.