I like the idea of self-discovery, but rather than tying the soul to a physical item it could be an internal journey. The soul's probably a lot more experienced than the current reincarnation, so you'd want to tap into its knowledge after you discover it, then learn to master it.
It's something like how Planescape's TNO learned from his previous selves, each with its own personality. In fact, the "evil" ones may try to reach out and influence you with promises of power, or even take over if you overestimated your willpower.
I didn't mean for the soul to be represented by an item b