Squidly

It floats through the still air of cellars and crypts, feeding on the ambient chill and the faint fears of enclosed dark places, and it propels itself in gentle drifting pulses of its ghostly mantle. Nearly weightless and nearly harmless, it relies on being difficult to grasp, slipping through fingers and walls alike as an insubstantial thing. When startled it releases a small puff of cold spectral ink that clouds the air and dims its glow, and it drifts away through the nearest wall while its pursuer is still swatting at the dark.