I think Leblanc might be after ancient magic from Shurima, Freljord and possibly other regions.

Cassiopeia is pretty much responsible for the rise of Shurima (possibly unknowingly). A person that was then inducted into the Black Rose directly after. That sounds kind of stupid though, don't you think? Cassiopeia is introduced to a secret ancient Noxian order, despite raising an ancient civilization back from the dead... With its ruler and the power he brings.... In a continent that Noxus has already disputed land on.

I think she was instructed to do so, an order from Leblanc. And Leblanc knew exactly what was going to happen. She knew Azir was going to be awoken. She wants the power that the sun disc holds. Possibly to steal its power for ascension?

But how could she possibly know every minute detail? The blood being spilled in the right spot, the specific person in all of Shurima being the one Cass kills being Sivir, Because it was prophesized. Leblanc was fulfilling that prophecy.

Ta’anari reached back over his shoulder and unslung the Chalicar. A murmur of shock rippled around the amphitheater at the sight of the ancient weapon. “You all remember this,” he said. “It is the weapon of Setaka, greatest and noblest of us all. Brought from beyond the mountain and raised aloft at Shurima’s birth. It is the blade that will one day be borne by Sivunas Alahair, the Bringer of Rains. In their hands it will be a weapon of great destruction, or a symbol of unity.”

While this isn't the specific prophecy, it's safe to assume there is another. It all makes sense and ties together really nicely.

And with Noxus targeting Freljord in pursuit of their True Ice, it seems to be that Leblanc is collecting ancient magic. But who knows what for...?