Two childhood friend. One is a crown prince to the Empire, the other is the illegitimate son of a noble family. Grew up together for ten years before cruelly being separated. After the cruel event, the crown prince became the Emperor that is revered while the illegitimate son is assumed dead by the public.

Two years passed, the assumed dead illegitimate son returned to the capital, except he had no memories of his life priors to two years ago. Fate decided to let the two meet again under a weird circumstances. Memories then slowly returning. Even feelings are slowly cultivated.

Sauce:

https://chrysanthemumgarden.com/novel-tl/teaht/ or https://m.tapas.io/series/The-Emperor-and-His-Thief/info