Complex sentence: In 449 BCE, the plebs seceded again to force the patricians to adopt the Twelve Tables, which, unlike the earlier secret laws, administed by the priests, these amounted to a written and published legal code.
Split into simpler sentences: In 449 BCE, the plebs seceded again to force the patricians to adopt the Twelve Tables. Unlike the earlier secret laws, that were administed by the priests, these new laws amounted to a written and published legal code.