One particular part of Karthus I still find a bit confusing...

Why is he still rivals with Soraka? I mean yeah Kindred is rivals to the other Shadow Champs due to them not being part of the cycle of life and death and what not, and it seems a little of the same reasoning with Karthus & Soraka, but there doesn't seem to be any proper explanation as to why Karthus and Soraka are rivals in the first place. Have they ever met? Have they ever fought? I mean what exactly in the lore (and no, I'm not talking about the fact that their ults completely counter each other) make it so that they are rivals? I'm just curious.