Thy spirit's sister, the lorn nightingale Mourns not her mate with such melodious pain; Not so the eagle, who like thee could scale Heaven, and could nourish in the sun's domain Her mighty youth with morning, doth complain, Soaring and screaming round her empty nest, As Albion wails for thee: the curse of Cain Light on his head who pierc'd thy innocent breast, And scar'd the angel soul that was its earthly guest!
Now, characters from Keats' poems show up to mourn his death. Here the poem becomes more clearly about Keats than about Adonais, whom Shelley has been using to represent the poet.
First, the nightingale, from Keats' "Ode to a Nightingale" shows up to mourn. The speaker calls the bird Keats' sister. Like Keats, he says, the bird could "scale heaven." He's using figurative language to elevate Keats.
He also mentions "Albion," which is the oldest name for the island of Britain. Keats also used the term in "On Sitting Down to Read King Lear Once Again". If readers weren't sure they were reading about Keats before this stanza, these references make Shelley's intention crystal-clear.
The speaker then calls down the curse of Cain on those who caused Keats' death. Cain is a character from the Old Testament who murdered his brother Abel. God was pretty angry, naturally, and banished Cain from his country.
The tone of the poem changes from mournful to angry in this stanza, because of this call for vengeance.