Soulshard of Malice
This item contains the soul of some malevolent being. Upon finding it, you immediately begin to hear the whispers. As you level up while carrying it, it levels up with you - feeding off of your life force . The whispers become voices which become screams in your mind - demanding to be released. Upon reaching item level 5 players gain the ability to make a pact and free whatever is inside.
Where this item leads it largely up to the DM. It may be a plot device, Object of Power, side quest, or even contain a Wandering Monster. It may also enable the unique multi-class Vessel of Ancient Evil.