Very difficult. The first that comes to my mind is 'Ur' from the 3e Weapons of Legacy supplement. In my 3e campaign I introduced many of WoL using my own rules and versions of them.
Ur is a biface, i.e. the earliest type of a 'handaxe'. It's closely associated with primal forces and the concept of 'hunting'. To unlock its powers three rituals are required. Each ritual requires the wielder to hunt and slay an animal single-handedly, first a stag, then a wolf and finally a bear.
The slain animal's spirit then becomes part of Ur, granting it additional powers (and turning it into an (increasingly) intelligent weapon). To use the powers fully the wielder has to allow an animal spirit to 'take over'.
So, after completing all three rituals you have an intelligent weapon with three different personalities, each with its own voice and goals that can come to the fore and guide the wielder's actions.
In my campaign it was initially wielded by a barbarian and after his demise it became the prized posession of the party's rogue.