Most love poems beg the beloved for mercy. Drayton's speaker aims higher and tries to bully the god of love himself. His lady's eyes have wounded him, but Cupid has been "so slack" that she feels nothing in return and even gets to "glory in my wrack", meaning she gloats over his ruin. The speaker's solution is to summon Cupid like a spirit and force him, by every oath he can think of, to make her love him. If Cupid refuses, the poem ends by telling him to be damned.
The insult in the first line sets the tone. "Purblind" means half-blind or dim-sighted, which is a sharper jab than the usual "blind Cupid." The speaker is not just describing the god but accusing him of bad aim and poor work. The word "wound" then ties the poem together. The lady's eyes have "wounded" him, he wants Cupid to "wound her heart", and the last item in the list of oaths is "all the wounds that ever thou hast giv'n." The poem begins and ends in injury, and the speaker wants the damage shared fairly.
The structure imitates a spell. After the opening quatrain, the poem runs through eight "by" phrases in lines 5 to 12, one pile-up of oaths in a single long sentence. The sequence is carefully ordered. It starts in the underworld, with "hellish Styx, by which the Thund'rer swears". The Thunderer is Jove, so this is the oath that binds even the king of the gods. Next come Hecate and "Proserpine's sad tears / When she was rapt to the infernal bower", with "rapt" meaning carried off. Between these dark figures sits Venus, "thy fair mother's unavoided power," a phrase that says her power cannot be escaped. The speaker then moves closer to Cupid's own life with "thine own loved Psyche" and the sacrificial "fires" blazing on his altars. Last come human things, "true lovers' sighs, vows, and desires." The oaths travel from hell to heaven to the speaker's own world, and each one adds pressure.
The form helps this build. The poem is a Shakespearean sonnet in regular iambic pentameter, rhymed ABAB CDCD EFEF GG. The alternating rhymes keep the list moving, and the quatrains each end with a pause that lets the next "By" begin a fresh push. Drayton also uses contractions like "Thund'rer," "heav'n" and "giv'n" to keep the ten-syllable line steady, so the rhythm sounds like a formal incantation, solemn and measured. Even the slightly archaic spelling "alters" for "altars" has a ritual feel to modern ears.
The couplet is where the tone turns. "I conjure thee by all that I have nam'd" gathers up the whole list like a lawyer summing up a case. Then comes the demand: "To make her love, or, Cupid, be thou damn'd." The line is full of commas, so it halts and stumbles at the moment of threat. The delayed "Cupid" also reads like a final, furious address. The god's name arrives only when the speaker has run out of patience. The rhyme of "nam'd" with "damn'd" is a neat touch, since it makes the act of naming lead straight to cursing.
There is a nice irony in the word "damn'd." Most of the oaths belong to classical myth, with Styx, Hecate and Proserpine, yet the threat uses a Christian word. A pagan god is being damned in the language of the Church. The effect is partly comic, and the whole poem balances on that edge. The speaker's pain seems real, but the over-the-top bullying of a god suggests he knows how extravagant he sounds. A lover who has to threaten a deity to win a lady is clearly powerless, and the boldness of the threat shows how helpless he is.
That mixture is what makes the sonnet worth reading. It takes the stock situation of a cruel mistress and unrequited love, and instead of pleading with the lady, it turns the complaint on the god responsible. Within fourteen lines it also shows how a sonnet can work as a piece of rhetoric, with a build-up of pressure and a sudden snap at the end. It reads like a speech by someone who has run out of gentler options.