I'm not sure what sort of thing you had in mind, but here's a magic longsword I made for my players which hopefully should be easy enough to adapt to your needs.

Telroth's Bane

Those with knowledge of martial history will know of Telroth as a renowned swordsman of centuries past. After years of winning duels without a single defeat, Telroth was finally killed in a duel with one of his apprentices; the swordsman made an uncharacteristic mistake, lunging too early and leaving his opponent free to run him through with his longsword. The sword has since become known as Telroth's Bane, the only weapon that could slay the famous warrior.

However, the true history of the blade is somewhat more complex. As a human living among elves and dwarves, Telroth became acutely aware of his impending death; all of his prowess would mean nothing if his knowledge died with him. Towards his later years, he became obsessed with a way of ensuring that his expertise would not die with him, leading him to explore increasingly drastic ways of preserving his legacy. Finally, Telroth made his decision: by allowing himself to be killed in battle by a specially-enchanted blade, Telroth could bind his spirit to the blade. After discretely paying for the services of a master smith and expert artificer, Telroth armed his apprentice with the blade and prepared himself for the fateful duel.

Telroth's Bane is a +1 longsword, although you can choose to delay adding the +1 property if you wish. Once attuned to the sword, the wielder can commune with the spirit of Telroth once per day during a rest. Telroth can provide advice and strategic insight to the player, although he is not omniscient and will not predict the future. This advice often, but not always, comes in the form of riddles or other cryptic speech, as Telroth believes no person should attain greatness without having to work for it. Unknown to the wielder, Telroth has goals of his own. These can take the form of specific tasks, opposition to a given group, or even a general alignment preference. As the player's actions move them closer to Telroth's goals, he becomes more helpful. If the wielder's actions oppose Telroth's goals, he will continue to provide advice, but will mix false information in with the good in an attempt to beguile and thwart the character. Telroth is initially reluctant to spell out his goals for the wielder, but may reveal more as he becomes positively disposed toward the player. You can choose to unlock additional properties as Telroth's wishes are fulfilled or keep it as a standard +1 longsword if that better fits your campaign.

Sorry about the wall of text there. Feel free to use the idea if you're interested; I tried to make a sword that provided fun RP potential in addition to combat benefits.