"I am sure you have heard the legends, Gallagher. The great weapon from the earliest Sith Wars- so long ago, that the name 'Sith' didn't mean anything in civilised space- they were fighting each other. A great war between two factions and an endless siege of a planet that could not be taken. But then the fabled Syphar n Kalar was used, and the planet fell immediately. The unstoppable weapon of the ancient Sith.
"A nonsense!"
"What is it that you think she is looking for, Gallagher? It's not a weapon. There's no respect in which it can be a weapon, because absolutely no-one knows what the darn thing- if a thing it was, if it existed, which it did not- was ever meant to be.
"She's looking for a legend and in that respect there is nothing to find. There are two major issues here for anyone who claims to look for such a thing. The first is the reason why anyone knows the term in the first place. The second is a simple logical analysis of the concept."
"The term 'Syphar n Kalar' comes from one source and one only- Daemora of Ciceronicus, who himself is old enough to be a legend. Back in the days of the very first Sith War, he ventured into Sith space- back in the days when people thought of the Sith as a distinct people, instead of the ideology it really was- gathered all the stories and legends about them he could, and wrote a book about it, called "A History of the Nations of the Sith."
"But Daemorra was a hack. He wasn't a historian of any sort- not even a journalist. He was on the take for money, and he filled his book with the most lurid and spectacular legends he could cobble together- no attribution, no sources... historically worthless.
"And within lay this story about the Syphar n Kalar. And that's literally it- just this one story about the taking of a planet. But that story took on a life of its own, and the Syphar n Kalar came to represent everything people feared about the Sith at the time- that they were unstoppable. Of course, the fact that they were very much stopped, in that they lost that war and each subsequent one, didn't enter into the legend. And in the centuries and millennia that followed, people decided they wanted to find this non-existent thing so that they might have the power of the Sith also. They looked for clues and sources and references... and they found many. But in every single case, it turned out the sources and clues they found were just people copying Daemorra's original text. Even the Sith themselves didn't actually have any records of this thing existing!
"And because people have juvenile imaginations, they assumed it was a big weapon. Even Daemorra said no such thing- he just says it was used, and it was unstoppable. It could have just been a Sith Lord of great power. Or an army. The name of a spaceship. A cult, a religion... a battle tactic. No-one knows... and no-one cares. It doesn't matter because it does not exist. It's just a fictional representation of people's fear of the Sith, and in being a legend it has created legends itself. At one point, any danger from the Sith was attributed to 'Syphar n Kalar'- the thing that would end your world. Just the Sith by another name.
"As for logic... this is simple. If the Sith had an unstoppable force of any description... why didn't they use it? Do you think they had a change of heart about unstoppable things?"
Your Jedi Lore knows that it doesn't exist, for the reasons Ascar outlined. You heard of it vaguely as the equivalent of a fairy tale in your youngest days. Anyone can just make up a name and say it is a super-weapon.
No unstoppable weapon or force was ever used by the Sith against the Republic, and even that statement is presuming some rational use of the term 'unstoppable' which is difficult to fathom.