In Claymore manga, chapter 45, The Witch's Maw, Part 5(21), Clare says that she has felt aura of yoma as strong and ominious as of Riful only one time before in her life.
But I don't remember her where she might have felt that aura. Among the awakened she met on her life which were shown in manga, Priscilla is the only one which have aura that powerful. But at that time, Clare was still human so she can't feel the aura.
As there are 3 case where she met the awakened(IIRC). One is the male awakened, second one is when she meets no 4 Ophelia, which was killed by Ophelia by herself and last one is the awakened Ophelia. Among three, I don't think anyone has as powerful aura as Riful.
Another option is, Clare is not talking about awakened but about the Claymore from number 1-5. As number 3 Galatea was with Clare in this fight, so she is out of the list. It can't be Number 1 Alicia and Number 2 Beth because they appear very late in manga. As remaining is number 5 Rafael, she does have enormous aura but they have never met each other. And she did felt something while walking away from Irene but not sure that was aura of Rafael or vanishing aura of Irene.
I tried out to find the time and the person but didn't find anything.
When did Clare felt the aura of yoma as strong and ominious as of Riful for the first time? Whose aura was that?