In the death-chamber for a moment Death, Sham'd by the presence of that living Might, Blush'd to annihilation, and the breath Revisited those lips, and Life's pale light Flash'd through those limbs, so late her dear delight. "Leave me not wild and drear and comfortless, As silent lightning leaves the starless night! Leave me not!" cried Urania: her distress Rous'd Death: Death rose and smil'd, and met her vain caress.
Death, the character, has returned, and she's embarrassed by the sight of Urania. Why? It could be because Death is the one that took Urania's son.
She's so embarrassed that she blushes herself into "annihilation." That's right, she blushes so hard that she disappears. (We almost did that once in junior high—awkward.)
Now that Death is gone, Adonais returns to life. Urania, of course, is elated. She begs him not to leave her again, in the "comfortless" world. Shelley uses another simile here, comparing the departure of Adonais to the departure of lightning in the night sky. It leaves in a flash.
Urania's delight makes so much noise that it rouses Death back from wherever she disappeared to (remember, Death is a woman in this poem). And of course Death takes back Adonais' life. All of Urania's caresses were in "vain." Nice try, Urania.
In this section, Urania's grief comes too late to save Adonais. She's a symbol of those that loved Keats' work, but who didn't defend him against criticism in time to save his life.