From The Chronicles of Tai

n the Third Month of the Second Year of Koushi, an Insurrection broke out in Bun Province. Aggrieved that War should have come to Tetsui, the Royal Tai set forth at the Head of the Imperial Army to subdue the Revolt.

In the Span of a single Month, His Highness disappeared in Ri'un, Bun Province, and a Meishoku occurred within the Imperial Palace. The Saiho vanished, throwing the Royal Ministers into Confusion.

Asen deceived the Court and donned the Mantle of a Pretender. Indulging in his Authority, Jou Asen (whose Name is Boku Kou), deftly deployed the Palace Guard of the Right and wielded the Black Arts as his Sword.

Having subjugated the Nine Provinces through his Inhumanity, He thus usurped the Throne.