He will awake no more, oh, never more! "Wake thou," cried Misery, "childless Mother, rise Out of thy sleep, and slake, in thy heart's core, A wound more fierce than his, with tears and sighs." And all the Dreams that watch'd Urania's eyes, And all the Echoes whom their sister's song Had held in holy silence, cried: "Arise!" Swift as a Thought by the snake Memory stung, From her ambrosial rest the fading Splendour sprung.
Adonais-Keats won't wake up ever again (in case you hadn't figured that out by now).
What's up with all the capitalization? Shelley is using some more personification; this time, Shelley makes "misery" into a character. Misery calls on Urania to wake up and figuratively "slake" (satisfy) her grief with "tears and sighs."
The speaker has resumed his requests for everyone to mourn. He took a break for a while, but now it looks like he wants us to get real sad again.
He also personifies Dreams and Echoes, which join him in asking everyone to wake up and do some mourning.
Splendour, once again a character, wakes from an "ambrosial" (sweet, delicious) sleep because of her memories of the youth.