Exar Kun is probably 2nd after Vitiate. That or Darth Ruin.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
__________________ There's a man goin' 'round takin' names.
An' he decides who to free and who to blame.
Everybody won't be treated all the same.
There'll be a golden ladder reaching down.
When the man comes around.
Marka Ragnos, who crowned Exar Kun and kept Vitiate as a mere vassal. Ushered in a Golden Age of the Sith in terms of stability and was immensely powerful, remaining unchallenged and undefeated until natural death.
__________________ There's a man goin' 'round takin' names.
An' he decides who to free and who to blame.
Everybody won't be treated all the same.
There'll be a golden ladder reaching down.
When the man comes around.
Yes I know that was before Nathema. But he's not saying "Ragnos before Vitiate hit puberty".
__________________ There's a man goin' 'round takin' names.
An' he decides who to free and who to blame.
Everybody won't be treated all the same.
There'll be a golden ladder reaching down.
When the man comes around.
Darth Ruin is a lot more relevant than Belia Darzu though, tbh.
Again, Exar Kun. If he and Freedon Nadd don't qualify as Ancient Sith, then Marka Ragnos, Tulak Hord, Naga Sadow, Karness Muur, they're all ridiculously powerful. Pick one at random and it's a legitimate answer, tbh.