I just started reading The Hero of Ages and I noticed that from a certain point Vin and other characters were mentioning Ruin by its name.
Maybe I missed it, but I can't recall them ever finding out the name of the creature that was trapped in the well.
I tried going back a few times, but all I could find was a quote about that the creature must never be released by Kwaan.
So in short: when did Vin et. al. find out that the creature that was released is called Ruin?