In his nightmare, the knight sees kings, princes, and warriors who tell him that he's fallen completely under the woman's power. The men, once strong, powerful, and chivalrous, are now starved, pale, and horrified. When he wakes up, the woman is nowhere to be seen. Like the men in his dream, the knight is doomed to wander pale and alone among the hillside, on a journey that never ends.
The climax of the poem occurs during the knight's terrifying nightmares in stanza 10. We learn that all of the men ensnared by the lady are of regal stature. This detail is crucial because it alerts us to the tensions between the woman's sexual liberation and the chivalrous, courtly tradition of the kings, princes, and warriors she seduces. Bound to a code of honor, these men would probably think twice before sleeping with a woman they met in the woods. However, their exhaustion, loneliness, and desire, combined with the woman's charm and forward nature, created the perfect storm for spontaneous, immediate, erotic satisfaction.
At first, the joy and pleasure these men experience testify to the value of a less straight-laced attitude to erotic love. But, as we know from the dream, falling for the lady proved disastrous: they woke up cold and alone, wandering forever in the woods, searching for a love that will never be found again. But the poem isn't simply condemning this kind of love; rather, it expresses a contradictory longing for both the courtly tradition's security and the intense passion of eroticism. For the knight, the woman's earlier emphasis on her "true" love for him certainly connected to an idea of stability and faithfulness; however, the association of sex with the woman's "faery-like," bewitching charms suggests the danger of erotic desire, and the uncertainty of these affairs.
Considering the poem's context in Keats' life—as well as the personal nature of much of his poetry—it's possible that the poet's allegorically-inflected knight's tale speaks to the frustrations and fears of his own relationship. Although Keats' love for Fanny Brawne was reciprocated, the couple faced difficult obstacles—first, Brawne's family was of a higher social class, and second, Keats' health was failing. Keats' proposals to Fanny Brawne were initially rejected because of his financial situation; Brawne and her family accepted when it became clear Keats was dying of tuberculosis, knowing well his winter in Italy would be his last. While scholars believe that Brawne and Keats never consummated their love, the powerful emotions expressed in their letters read much like the speaker's passion for the lady.
Giving in to sexual desire could mean losing love and experiencing profound emotional pain—but resisting this desire can be just as painful, and just as preoccupying as love's loss. Is it better to wander cold and alone, stuck somewhere in a dream-space where time appears to stop, with only the memory of this love? Or should one turn away from love and hold fast to tradition, regardless of how strong one's desire may be and how lonely one may feel? Even though the knight was bewitched by the woman, the pleasure and joy he felt during their brief, erotic relationship was real. The woman is no longer physically present at the end of the poem, but she still holds countless men "in thrall," or captive beneath her power.
When the knight ends his story, the speaker from the first stanza has yet to meet the lady in the meadows. What choice will he make when they eventually cross paths? Will he make it through the woods with his heart intact? Or will he be like the other men lost in the woods, alone and bloodless, only capable of reliving his story as a warning?