It's not hard to kill someone with a knife. I can easily sink my throwing knives into a tree trunk even without a lot of power behind my throws. And, being cut with a knife HURTS. Especially when it slips from my hands. If it began raining knives...
People outside would be injured or they'd die, and houses would suffer huge damage (broken shingles, cracked overhangs perfect for termites, etc.)
If you mean that the character was in the arena when the rains began, then most likely, unless he/she seeks immediate shelter under a thick (extremely) canopy of branches and leaves or an actual shelter (if made of wood, a higher roof), he/she would be severely injured. Unless, again, he/she is miraculously able to dodge all those falling knives. :] Wouldn't that be fun? Dodgeball with your life as the stakes?