"Slam Shut" or just a nick name for 'suddenly go rigid' -- "Hardness" is a 3E term I can go with. Imagine it just calls upon a little bit of energy and defends itself a little.
Hardness
Encounter - Immediate Interrupt
Gain resist 5 weapon until end of ~'s next turn. (Small object or smaller.)
Gain resist 10 (medium), or 15 (large).
If you go with the above poltergeist concept, I like the idea of enchanting 4 small items, 2 medium items, and 1 large item. Disenchant the first few items after less hit points, giving it a sense of "small items can't take damage for long" and they're disposable. Small/tiny objects should gain a great bonus to evasion and difficult to damage effectively. Resist 2-3 weapon should be enough. Perhaps using the above power with a recharge when a new item is possessed...
First quarter of hp -- (one surge's value)
one sixteenth = one quarter of 1/4 hp = 1 small item
one sixteenth (again, 1 small item)
one eighth = one half of 1/4 hp = 2 small items, together
Second quarter of hp -- (total minus surge value down to bloodied)
one eighth = one half of 1/4 hp = 1 medium item. Medium objects gain hardness 5.
one eighth (again, 1 medium item)
Third quarter of hp -- (bloodied down to surge value)
Possess two medium objects. Hardness power affects both objects, 5 or 10 at your option. Since we're getting low on hp, and are definitely low, might as well consider 10 to both objects as a 'last resort'.
Final quarter of hp -- (surge/critical value down to 0)
Possess a large object. Large object gains hardness 10 or 15, your choice. 15 is a good way to make it hard to damage for one turn straight -- and negates one attack. Since our spirit's already at critical, why not give it a last breath attack?
I figure giving this, say, 100 hit points as a spirit, call it a lurker, and go by 6, 6, 13, 12, 13, 25, 25... (small, small, 2 small, medium, medium, 2 medium, 1 large).
Also, figure in that this creature uses "hardness" the first time it gets hit in its new form, unless the hit's damage was negligible (2-4 damage when 12-13 hp). Improve damage in each form size, 1d4+1, 1d6+3, 1d8+5. Keep it heroic.
Flying small objects can attack Reflex, optionally. Large object can attack Fortitude optionally. Medium objects can attack Reflex or Fortitude, or just stick to Armor. Think about how it might focus on combatants, then -- small objects attack low Reflexes, large object goes after low fortitude, medium objects go after low Armor...
Let small objects fly-hover. Let medium objects fly 2-3 (hover) or have a base speed. If it's an animated weapon, use its effect instead, add a modifier of choice. I see animated daggers, animated spear, animated polearm... who knows... you could add combat maneuvers to a given spirit...
What are you thinking of doing?