I might be wrong, so you are free to correct me,
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but as I view It, The "TRUE SITH" made the mandalorians attack the republic, then Revan beat the Mandalorians, and on Malachor 5 found out about the TRUE SITH and why theuy are so dangerous, as it apparently is said that Trayus academy was built by yhe TRUE SITH. So Revan realisd they are a threat to the galaxy, and became the dark lord of the sith out of necesity to control the republic through conquer, and thus make an army good enough to fight the true sith. HE became the Dark Lord of the SITH Empire that was known to the galaxy (so in other words, NOT the TRUE SITH) a he realised that he could not takre control of the republic otherwise, (notice, he didn't destroy industry and infrastructure on the planets he conquered.
If the are any mistakes in my understanding, please feel free to correct me