Legends introduced countless Sith, with stories set in the Old Republic featuring the Sith at the height of their power, long before the Rule of Two was instituted or the dark-siders were forced to go underground.

Source: https://www.cbr.com/star-wars-canon-sith-lords-outside-skywalker-saga/

The most infamous Thought Bomb was used by Lord Kaan at the end of the Ruusan Campaign, leading the last living Sith, Darth Bane, to create the Rule of Two and hide the Sith Order’s survival.

Source: https://screenrant.com/star-wars-legends-jedi-dark-side-force-powers-canon/