This Mercy is among the "dullest" of the swords, yet perhaps it offers one subtle trick (if unreliable) trick courtesy of its once-per-turn effect. The ability to force a re-roll if attacked gets around the normal limitations associated with Probability Control (modulo my understanding of "being attacked" and "becoming a target"). This isn't the first effect I'd be looking for from a Sword, but if I was fielding a force that I thought might be susceptible to something like Quake or Energy Explosion, and I had an already defensively-minded Sword Bearer (or a low-cost one I wasn't planning to get a lot of offense from) that could be adjacent to my other figures, I might consider using this Sword as a little extra (defensive) insurance.