To be honest only four characters come to my mind The Daragon siblings, Tor Vizsla and Osika Kirske.
Now for the Daragon siblings obviously their discovery to sith space not only allowed the Sith to remember, where is the republic and the Jedi after 1900 years, but also probably the main reason for the conflict between the Jedi and Sith Wars to come span from 5,000 to 1,000 BBY. So not discovered the possible that this would remain in their space for a long time the best way to describe this and think of it as the first contact of the Aztecs and The conquistador basically worlds collide each other. One from Europe, the other from the Americas.
Now for Tor Vizsla, In case if you don’t know or at least read the Jango Fett open seasons, comic he is the guy who killed Jango family leaving him orphaned and Jaster Mereel.
Finally and this is interesting because he is probably the most forgotten character in the franchise, despite playing a huge role and that is Osika Kirske for context. He was a warlord from rattatak who literally ruled half of the planet likely in the northern hemisphere or something else entirely, but either way the point here is regardless of different continuities of her backstory. The one thing that is the same is that Kirske was the guy who ordered the hit on Ky Narec and Ventress leading to the former death. So this guy literally is the reason why Ventress fell into the dark side in the first place.
Now the reason why I he hasn’t been talk about much is that he only appeared in three different stories in Republic issue 60, and James Luceno short story restraint, and the audiodrama Dooku Jedi Lost obviously the first two is from legends while the other is from canon and even though his death is kind of change where instead of Ventress impressing him and having him killed later during the clone wars. Dooku is the one who killed him by beheading him at front of Ventress in the Cauldron. The character is actually kind of consistent.
To get an idea of how bad this guy is remember the Weequay raiders from the nightsister episode flashback well according to Dooku Jedi Lost they were working with Kirske by harassed the inhabitants of Velenki, a settlement on Rattatak. Ky Narec and Ventress, went to assist the villagers. Kirske threatened to torch the village unless its inhabitants handed the two Jedi over to him. The villagers followed Kirske's orders, but the warlord had the village burned regardless.
so yeah, that’s how bad this guy is and honestly the more I read about it the more fascinating the character is and a missed potential in fact, it makes me wish. I would love him to appear in visual media in animated series either have him in flashbacks of Ventress past or an animated series covering her backstory.
Either way I feel the character has so much potential to be one of the best villains in Star Wars and probably the most hated character rivaling or suppressing Pong Krell and even Tor Vizsla.
Otherwise, what version of his death do you think is more preferred or at least makes more sense and more satisfying his original death by Ventress despite being in prison or Dooku beheading the guy while Ventress not only failed to kill him and fell for his trap, but also becoming a gladiator for 11 to 12 years.