User blog comment:Sarkis KHorozian/Karthus edits/@comment-4080931-20120419223251/@comment-84.229.75.144-20120421080731

You are wrong! Nunu's ulti also slows the enemy for duration of the spell while Karthus's doesn't, so we need to make Requiem do true damage. Moreover, Sivir and Noc can block it with spell shield which is plain unfair (blocking ulti with regular ability!) so we need to reduce Requiem's CD to like 10-12 seconds