The leprous corpse, touch'd by this spirit tender, Exhales itself in flowers of gentle breath; Like incarnations of the stars, when splendour Is chang'd to fragrance, they illumine death And mock the merry worm that wakes beneath; Nought we know, dies. Shall that alone which knows Be as a sword consum'd before the sheath By sightless lightning?—the intense atom glows A moment, then is quench'd in a most cold repose.
This stanza serves as contrast with the previous stanza. Now, Adonais-Keats is described as a "leprous corpse."
Leprosy is a disease that causes tissues to rot; basically, our speaker is (pretty graphically) describing the process of decay. That's quite a contrast to the way the body was earlier described. It's not so beautiful and serene, now, is it?
But, have hope, Shmoopers: the stream of life is coming to save the corpse. The "spirit" of this stream comes and fills the body, and it "illumines" (lights up) the body.
It's nature; it turns the body, buried in the earth, into something living; it becomes part of the ecosystem again.
He compares, using a simile, the process of nature to the way the light of the stars are cast down to earth. Their dying light is changed into something brilliant that we see and enjoy here on earth.