Starsoul Flask II

Starsoul Flasks are made primarily from Rockthistle Seeds, which give off a soothing aroma.
The Creed's priests also infuse the seeds with a small amount of Engram, which is why flickering motes appear inside the vial.
It's the gentle nourishment of Engram, paired with Rockthistle's sedative effect, that lets Starsoul Flasks relieve people wounded by Illusory Plague to a certain extent.

But overall, their effects are overstated. Starsoul Flasks are basically a "placebo" and can't truly eradicate Illusory Plague.