Kingfisher Feathers (слэш)

With an almost trance-like detachment, Wei Wuxian touched his own neck, his fingers skimming over the fresh mark. The bite wound had stopped bleeding, although he had no doubts it would open again if agitated.
Bonded.
He was bonded for life.
"Shit," he whispered. He looked over at the sleeping form of Lan Wangji—the Second Prince of Gusu and, until his brother was found, the sole heir to the throne. "Oh, shit. Lan Qiren is going to kill me."
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Lan Wangji goes into a fevered rut and accidentally bonds with Wei Wuxian. When they next meet, he remembers none of it, and Wei Wuxian is determined to keep the bond a secret—even when he's sent to the Cloud Recesses to be a consort in Lan Wangji's harem.
(tl;dr concubine!wwx is already married to emperor!lwj, who has no idea. drama ensues.)