As with all things D&D, consider looking to Tolkien for inspiration. A powerful force for darkness creates magic items that grant power, but at the same time expose the owner to danger? Sounds like a ringer of a story.
At first the Warlock feels like he just has a magic weapon. It allows him to blast eldritch might and curse enemies... neat! But then he notices something when his enemies fall. As they die, his blade is draining them of something. He can see it in their eyes... They don't just die... Their soul is ... taken? Devoured? Then the voices begin. Promising him more power as he tells more enemies. It only takes the souls of the Damned it says. Their actions in life sealed their fate. The weapon is just carrying out the Will of the Gods. And it isn't even the real blade... just a lesser blade in the service of the master blade. They should go find the true blade... The story continues to escalate until the PCs have to decide whether to oppose the force because it is evil, or use it because they need it to stop greater evils.
Or the PC inherited the blade from his grandfather, a powerful master of might and magic who died many years ago, but who's body was only found recently. But, as soon as the PC lifts her blade, the weapon reveals itself it discovers that the weapon was the source of their grandfather's power... and is now so weak from not feeding that it can only offer meager power to the new owner. But, the more she uses the blade, the more it will grow in power and share that power with her... It just needs to heal, so it will let her decide how to use that power... at first... But once it has regained some power it will start to suggest options. Then it will ask her for help with ancient unfinished tasks. Then it will lie about other things to keep her doing its bidding willingly...until finally she starts to experience blackouts. And then there is the whole diving into two, etc....