In need of suggestions how elegantly to play corrupting influence of the sword on player, who currently owns it.
He's character is an elf-magus, risen in human society, by his elven parents, who where servants of some rich human family.
In 3.5 I ran an intelligent sword with our Warpriest. I spent approximately 3 sessions talking with him about how impressive he was. I called him "My lord", in the passed notes to him. The weapon used its magic ability to alter his appearance so that his sword and armor appeared plated in gold, and that he stood a few inches taller so that he would be more impressive to friend and foe alike.
The player himself was a strong strategist, and a Cleric of a War deity. When discussions came, I would tell him that he was the one most fit to make the call, and that he needed to assert himself. Then I would encourage him, that they needed to see demonstrations of his power in order to understand how much greater he had become. Then he had to start making Will saves, or he would be offended if they didn't call him, "My lord". By the end, he had adopted the moniker "Lord Brendan the Majestic" or some other pretentious thing.
The thing I liked about it was that, though the player realized fairly early on that the sword he'd adopted might have been trying to negatively persuade his character, he also saw how insidious it was, and how it played on the character's desire to be powerful and worthy.