Complex sentence: Concurrently, Lü Bu has also returned after Dong Zhuo sent him to choose a new steed for Cao Cao, so Cao escapes from the capital Luoyang under the pretext of test-riding the horse.
Split into simpler sentences: Concurrently, Lü Bu has also returned after Dong sent him to choose a new steed for Cao. So Cao escapes from the capital, Luoyang, under the pretext of test-riding the horse.