Which makes me think of....
Arrow, Weakfinder: This enchanted arrow will automatically aim at the opponent with the lowest hit point value. An attack roll must still be done in order to hit it.
Creating game materials? Monsters, spells, classes, adventures? This is the place!
Ah, I didn't read the original text carefully enough and thought the dice roll applied to damage (I guess I'm still think in terms of thac0/to hit as the terms and my mind matched attack bonus with damage). The weapon makes a lot more sense now.