Ashbringer:
Commander Eligor Dawnbringer: Yet you think that recovering Ashbringer will somehow turn the tide of battle? Let me let you in on a little secret, Commander. The power of the Ashbringer came from the man who would wield it... I was there, Commander.
I watched him burn legions of undead in righteous fire before he would even unsheathe the blade. Alas, your grasp on history is... lacking.
Frostmourne:
The Lich King uses this ability to try and corrupt the wielder over time. This has several effects. An individual who wields Frostmourne will not part with it willingly. Over time the person will go from good to neutral and finally to evil. A non-undead evil wielder will then become undead.
Finally the sword is able to suck the being's soul into the sword. Frostmourne then retains the memories and skills of its victim. In fact as long as the wielder's original body remains within one mile of Frostmourne, the sword maintains control of the body, and he may not be completely aware that his consciousness has shifted to the weapon. Beyond one mile, the sword cannot control the wielder's body, and the wielder of course finally becomes quite aware of his current state (if he is not already).
-wowwiki