I don't understand what こと means in this sentence:
こんな 恐ろしい 事 は 聞いた こと が ない。
I initially thought it was a nominalizer, but nominalizers must be used with verbs at the neutral form, and also, only the nominalizer の but not こと can be used with perception verbs.
Intuitively, I guess it means something like: 'the fact that I heard something so horrible doesn't exist' (ない). But I am not sure.