Hmm bursting into flames could work, but that's up to Charity since she'll be the one taking the sword XD
Also, paralysis is a nice idea. I was thinking something along that line, or a some sort of stasis. The rune to slower the trap could hold for enough time to prevent it from activating when first unsheathing it, but once the effect wears off, drawing the sword from the scabbard other times would activate the enchantment all over again and trap whoever did it