I like the idea of a fell drain cloud of knives. In fact, you could persist that and do it all day as a free action. How many spells could be persisted like that? I can imagine a character with fell drain persistent cloud of knives, fell drain persistent holy star, and fell drain persistent lightning ring. Doesn't storm tower give you an at-will attack as well? What options would a divine character have for reducing the metamagic cost of Fell Drain, apart from the Dweomerkeeper capstone?