Simplify the following text for a Grade 8 native English-speaking person.
Original: Silla , which had accomplished an incomplete unification of the Three Kingdoms of Korea in 668 , weakened and lost control over local lords during the end of the 9th century .
Simplified: Silla had accomplished an unification of the Three Kingdoms of Korea in 668 . But it weakened as local lords achieved power during the end of the 9th century .