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The Law of the Twelve Tablets did not change the form of Roman government. It was compiled in 451 BC and 450 BC; That is, 58 and 59 years into the Roman Republic which lasted for 482 years (509 BC-27 BC)
The phrasing is unclear. The form of government which preceded the Roman Republic was the Roman Monarchy. The Republic was followed by rule by emperors. However, the Republic did not pass this on to future civilisations because: 1) it was the collapse of the Republic, rather than some sort of handover; 2) no future civilisation tried to replicate Rome's period of rule by emperors or, for that matter, the Roman Monarchy.
In 509 BC the Roman republic was established. However it took roughly another year for a workable form to be put in place.
It's a tossup between Julius Caesar and Octavian. Julius Caesar was the last single leader of the Roman republic. However the republic did not abruptly end at his death. The second triumvirate was formed, which was a legally appointed trio with each man having equal power. When the triumvirate fell apart and Octavian came out on top, he technically became the last leader of the republic, making it much easier for him to form the principate.
A triumvirate was a rule by three men. This form of government took place twice during the last phase of the Roman republic.