Another interesting idea Crothian...you must be a very, very advanced AI program to respond so often and quickly with such creative ideas
I was thinking about the history of the ring...so here's a couple of options:
1. The creator is the inhabitor of the ring. He was attempting to make a phylactery and completely botched...it kept him alive in an extremely limited capacity and he's learned to use what little abilities he has. The botch could have been caused by disinformation planted by a rival or just a common mistake.
2. The creator was a good being/good deity. The purpose of the ring was an eternal prison for the evil soul. Unfortunately (through lack of sight, a loose grasp of how powerful the soul actually was) the prison kept the soul contained but able to manipulate reality (with some practice). The soul learned its limits within the ring and constantly tests them.
3. The creator (and inhabitor) was already a lich. He/She was testing if his soul could be split into more than just one phylactery...so as to assure his permanence indefinitely. Of course, there was an error in the experiment...and his soul was split into two. One piece is in a phylactery somewhere else...probably the lost-to-time former layer of said creator (on any plane).
So now you have both "good" and "evil" reasons for the artifact's origin. Of course, the ring serves as a reverse phylactery. Once broken, it releases the being back into the world. Options 1 & 2 allow the creature to return as a living being. Option 3 permits the return of an undead being (assuming the original phylactery is still intact).
If you go with Option 3, the ring may also encourage its owner to seek out the original phylactery. Then the party may have to question why they quested through hundreds of miles of underground dungeon all for a "lucky" rabbits foot.

You may wish the artifact to have the ability to sense its phylactery (or phylacteries in general). If it can sense all phylacteries...bingo you've got yourself quite the hook there. The imprisoned soul may wish to destroy all the phylacteries belonging to other liches to make sure he reigns supreme when he does reemerge.
And hey, if you're feeling corny, you could name said spirit Themal. And then you'll have
one ring to bind Themal.
Either way, the balance you need to worry about is truly making sure the other players don't feel left out as stated earlier.
And another thought, as the ring regains sentience, it could "possess" (a.k.a.
manifest) like a ghost through its user. The user could also suffer nightmares of assumed past "lives"...acts truly committed by the ring's prisoner...etc.
All-in-all, it should produce some nice adventures and possibly wind up a plot thread that the party focuses upon heavily....and in doing so neglects the "real" plot and end up all the more f-cked for it.
~Fune