Designing an item that's only useful in one extremely rare circumstance is a waste of time, and of a unique item. 99 times out of 100, you'll have no use when it comes up, and when you do need it, you can't rely on it coming up. But hey, painboots.
Needle-Soled Boots
Description: Mind-searing pain can be a powerful motivator.
Protection: 2
Durability: 100%
Speed: +0%
Knockback: -25%
Special: You lose 1 hp when you put them on, and 1 hp every 50 turns (5 seconds) afterwards. You gain 1 effective level in each of Hellrunner, Finesse, and Reloader. These effective levels do not affect trait selection in any way and go away when you remove the boots.
Something a little more serious
Stim Armor
Description: Armor with an adrenaline pump? That seems like overkill...
Protection: 2
Durability: 100%
Speed: +0%
Knockback: +0%
Special: Increases berserk countdown by 10% (berserk packs give 55, Berserker gives 22). When damaged, there is a chance (5% per durability lost) that you go berserk for 20 rounds. When destroyed, you go berserk for 20 rounds.
Something a little less serious
Security blanket
Unique armor
Description: It doesn't matter how dirty or tattered your Binky gets, you love it more than ever!
Protection: 3
Durability: 100%
Speed: -10%
Knockback: +0%
Special: Instead of decreasing when damaged, the Security Blanket's protection increases, to 5 at 49%, and 7 at 24%.
And something too silly to mention but I'm going to mention it anyway
Lost Soles
Unique boots
Description: You found a couple of lost souls asleep on the ground, and you tied them to your feet for some reason?
Protection: 1
Durability: Unlimited
Speed: +50% (or +100%, whichever one means half movement cost)
Knockback: +0%
Special: If one or more enemies are in vision, you must end a move closer to the closest enemy or else you get the message "Your boots balk!" and instead of a speed bonus, you have a -50% speed penalty. If you try to remove them, you are prompted "Really free the lost souls? Y/n" and if you choose Y, the item is destroyed and two lost souls are spawned adjacent to you.