A.N. I'll probably finish this around chapter 100. Give or take a chapter or two.
Chapter 88 - Understanding Magic
With their first question, Raven gained complete knowledge of the source of Salem's immortality. It was a theory that she proposed to Rudy when she first learned about his own curse of immortality.
What exactly made Rudy so special? The obvious answer was Magic, but more specifically? Why is it that only a select few could use Magic.
At its source, all Magic originated from two sources. The first source is Ozma, with his Maidens and the siblings Qrow and Raven. And the other source, Salem. Who controls the Grimm and technically Rudy.
So what made these two individuals special? Rudy knew the answer to this.
It was the Gods.
Raven theorized that the Gods are the source of all Magic. Without the Gods, those living on Remnant no longer have access to Magic and its power. The only exceptions were those whom they paid personal attention to, which were Ozma and Salem.
And then there was Raven's underlying theory of the source of their abilities. The Gods were special because their power was that of Divine making. Their divinity left with them when the two Gods left Remnant, but they did leave behind a small portion of their power.
Yes, the Relics. The four Relics not only contain an aspect of humanity but are also the last source of Magic itself. Rudy could argue that it might even be the source of Aura, Semblances, and Dust as well.
So how does this all tie to Raven's question to the Relic of Knowledge? Well... it's simple.
To kill Salem, you must first remove the source of her immortality.
Which meant they had to destroy all four Relics.