Long has the question of truth been pondered. Long has this been a folly few recognise in time. A killer, a soldier even with a sword is one thing. But a killer who thinks he has found a definite truth, which he decides to fight for, is the most dangerous man in the world.
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