The warrior's footfalls echoed on the cobbles. His breath steamed in the cold, frosty air. He heard the frantic movement of assailants in the shadowed alley shuffling to surround him.
The warrior laughed. It was a bitter, dry laugh. "It won't do you any good. I feel no pain. Prepare to meet Balor."
The creation of intelligent magical weapons is not an exact science and there are occasional failures, as such think of causing a little more stress to your players by "rewarding" them with an insane magical weapon
Agrophobic, refuses to come out of the scabbard without a strength check to draw it.
Haemaphobic: afraid of blood, fight at a penalty against any blooded creatures but really good against undead, elementals etc
Schizophrenic, not just one mind in your sword, lets have half a dozen minds in there, all different and bound to cause confusion
the possibilities are endless