Note that this poem is in the classic form of the French sonnet, which differs slightly in arrangement from both the Petrarchan and the English sonnets. Ideally, there are two initial verses of four lines each (octet) and then two verses of three lines each (sestet). And of course, there's a fairly regular rhyme scheme, just as in the classical English sonnet. Alternate ABAB rhymes in the two parts of the octet, and the sestet's pattern is: CDD CEE.
Je suis le Ténébreux, – le Veuf, – l’Inconsolé,
Le Prince d’Aquitaine à la Tour abolie :
Ma seule Étoile est morte, – et mon luth constellé
Porte le Soleil noir de la Mélancolie.
"I am the man of shadows, - the widower, - the unconsoled,
The Prince of Aquitaine of the ruined Tower:
My only Star is dead, - and my starry lute
Carries the black Sun of Melancholy."
The "Prince of Aquitaine" is not a random image (no image is in this poem); it refers to Godfried d'Aquitaine, a medieval lord famous for his misfortunes, who came from the same provence Nerval came from. The connection is not crucial to understanding the poem, but it does at least give you an idea of why he uses that image in particular to emphasize his desolation. And why is he desolate, why a widower and unconsoled? His "only star is dead" - so there's been a death of some sort, figurative or literal, a loss of someone who would, perhaps, have consoled him. Now his "starry lute" - a musical instrument equipped to bring light to the darkness (and, as Nerval makes explicit by the end of the poem, a reference to Orpheus) brings only the "black sun of melancholy", "black sun" being a technical term from alchemy to describe a powerful "negative light" (though the idea is quite absurd) that is not merely the absence of light, but its inverse. The imagery as a whole in this stanza is quite easy to follow all in all, giving us a picture of a sort of "dark night of the soul" or of a descent into a realm of overwhelming darkness.
Dans la nuit du Tombeau, Toi qui m’as consolé,
Rends-moi le Pausilippe et la mer d’Italie,
La fleur qui plaisait tant à mon cœur désolé,
Et la treille où le Pampre à la Rose s’allie.
"In the night of the Tomb, You who consoled me,
Give me back Posilipo and the Italian sea,
The flower which pleased my desolate heart,
And the trellis where the Vine and the Rome are united."
Posilipo and the Italian sea are partly autobiographical references to a summer he spent with an Englishwoman in a town near Naples. Yet like the Prince of Aquitaine imagery, the self-referential aspect of this line is really not the main point. There is, of course, the larger image (almost cliched in Nerval's time as well as ours) of the warm, brilliant south as another world in which consolation was possible. The syntactical confusion in the first line of this stanza - "Dans la nuit du Tombeau, Toi qui m’as consolé" - forces us to wonder whether the speaker is referring to himself as metaphorically dead, or to the one who consoled him as literally dead. Or perhaps both, the former as a result of the latter? And we also get a confirmation here of what the phrase "the unconsoled" (as opposed to "the unconsolable" or something tantamount) made us suspect in the first stanza: there was once someone who had consoled him, and she is now gone, just as the comfort of the time in Italy, of the lost flower are gone. He longs for a return of the trellis which made possible the union of two different yet complementary beings - the vine and the rose.
Suis-je Amour ou Phébus ?... Lusignan ou Biron ?
Mon front est rouge encor du baiser de la Reine ;
J’ai rêvé dans la Grotte où nage la sirène...
"Am I Cupid or Phebus? ...Lusignan or Biron?
My forehead is still red from the kiss of the Queen;
I have dreamed in the Grotto where the mermaids sing..."
Now comes the turn in the poem from the speaker's lamentation of his loss to his meditation on how to respond to it and on his role as a poet. Not how the opening of the sestet reverses the declarative "je suis" at the beginning of the octet to the questioning "suis-je?". The references to mythology and folktale only hinted at in the previous eight lines now become explicit, as he explores characters from both Greek mythology and French legend as possible analogues to himself. Is he Cupid, the god of passion and eros or Phoebus, god of the sun, of rationality, of light? Is he Lusignan, husband of the fairy Melusine, or Biron, a French hero whose name also recalls the English poet? The tension between the enchantment of eros and the appeal of rationality and love of higher order is reemphazied in the two following lines, where the "kiss of the queen" merges with his dream of a mysterious grotto filled with mermaids. It's also important to remember that both Cupid and Lusignan lost their loves because of a transgression - Cupid because his wife, Psyche, looked at his face against his command, and Lusignan because he saw his wife bathing and thus discovered that she was really a mermaid. Apollo and Biron, on the other hand, are the pursuers of light, yet have no record of seeing what must not be seen; and as poets, they have the ability to do exactly what Nerval is trying to do in this verse: recall a joyful time that has now disappeared from physical sight.
Et j’ai deux fois vainqueur traversé l’Achéron :
Modulant tour à tour sur la lyre d’Orphée
Les soupirs de la Sainte et les cris de la Fée.
"I have twice crossed the Acheron, victorious:
Modulating by turns on Orpheus' lyre
The sighs of the Saint and the calls of the Fairy."
Now we get the explicit connection to Orpheus, the lute/lyre player, who crossed the Acheron in an attempt to retrieve his wife from Hades but who on the victorious return lost her once more, and decisively this time. As he goes, the melody of the lyre, mirroring the flow of the poem, "modulates" between the "sighs of the Saint" and the "cries of the Fairy": contrasting the dark night of the soul described in the octet with the alluring calls of myth (sestet) which may or may not turn out to be consoling.
It's easy to see what aspects of the Orpheus story Nerval wants to emphasize after having seen the contrast between the mythological figures of the previous tercet. He's fascinated by the way the poet seemingly has the ability to evoke his lost love (or whatever he's describing) in a manner so real as to make it almost present once more. Yet there's always the danger that as a human, under the influence of eros and other no less strong desires (curiosity and lack of trust in particular), he will "look back" against the command of the form, and realize that in seeking to ensure the presence of his loved one, he will seal his loss. You cannot be conscious of the fiction of the poem if it is to really make present what you are hoping to regain.