Personally, I don't think Odd Keystone should be implimented. Consider:
1. All current signature items fall into 4 ingame-based categories: they are used to evolve a Pokemon; they are used to make baby pokemon; they serve as the catalyst of a form change; they have an in-game battle effect. The Odd Keystone fails to fall into any of these categories. It doesn't evolve Spiritomb, Spiritomb doesn't have a baby form, Spiritomb has no alternate formes, and the Odd Keystone has no ingame battle effect. Even items such as the Soothe Bell have basis in the fact that they increase friendship, allowing Pokemon that evolve by that method to evolve faster ingame.
2. The item is either outclassed or illogical, bar the pressure effect, even in conjunction with another affect. Raising a single defensive stat a single rank is obviously outclassed by Everstone, which raises its Def and SDef along with boosting the BAP of its STABs. While a boosted offensive stat may give a better damage output than Everstone, it is in turn outclassed by items such as the Life Orb and Expert Belt. Additionally, Spiritomb is a balanced pokemon. Its special and physical sides are statistically equal, which would lead to the question of which stat to boost, and why does it have precedence over the other? In regards to fairy immunity, it is illogical. What flavor-based or mechanic reason does Spiritomb have to suddenly gain a free immunity? This is an in justified buff with absolutely no reason other than to buff. Even Dusknoir's Reaper Cloth, which gives it levitate, has a reason in the fact that Dusknoir is a distinctly floating Pokemon.
3. If we were to implement Odd Keystone, what other items would we have to consider that shouldn't be implimented. Big Nuggets? Training Bags? Lava Cookies?
Before we consider what effect it will have, consider why we want to impliment this as an item. Why does it need to exist when we already have a perfectly viable "signature item" for Spiritomb to use in the form of Everstone.
Also, as a side note, ED, if you were more willing to apply a nature that reduces speed or defenses, maybe boosting its offenses wouldn't be such a problem. I feel like this reluctance to apply certain natures is influencing your suggestion in a way the majority doesn't necessarily agree with. For example if I had a Spiritomb, I'd be perfectly happy running a Brave or Quiet nature, an using Everstone to compensate for my subpar HP.