This small, thin object is pointed on either end, with a grip in the center. It's made of a material that looks like silver but is springy and resilient - to the knowing eye it is immediately identifiable as goblincraft, marking it as a foul object. In the hands of a Small creature it can be wielded like a melee weapon - in larger hands it isn't very effective this way and works better as a javelin.
Trained goblins know how to use the magic properties of the Spine, but for most it is random. If it strikes a target, it will always unleash a point of electrical damage and absorb a point (but only from living creatures. Constructs, undead, or anything else lacking a central nervous system can be shocked but can't also be drained). It will absorb an additional point each round until withdrawn. In other words, a strike does whatever physical damage the Spine does (1d4) + 1 point of shock + 1 point is stolen if possible.
The absorbed HP is stored in the Spine, to be released as shock damage in a 1/1 ratio. Those trained in its use, or who study it long enough, learn to control this so they can strike without releasing HP (to store more of a charge), and even control how much HP is released in order to cause more damage or hold back in order to incapacitate.
Whenever a thrown Spine hits with a natural 20, it has tapped directly into the nervous system of the target. It goes apeshit. The Spine drains the victim of 3d6 HP, but immediately arcs it as shock damage to every creature within 15 feet (that is conductive). The HP is split evenly among them as shock damage. The Spine's owner and her allies are not immune to this effect. A trained user can choose to use a Will saving throw, difficulty of 10, to prevent this. This effect does NOT occur if the weapon is used in melee (physical contact fully asserts one's will over the Spine).
When the Spine holds a full charge, it will attempt to read the mind of the next creature to wield it. If successful, it will use all its energy to change shape into the type of weapon they are most experienced or skilled with. At first, it can only stay within the same relative size (if they use a longbow, it will be a short bow, if they use a longsword, it will be a shortsword, etc.) The Spine is then no longer the Spine - it's a goblincraft version of whatever weapon, doing the standard damage, etc. It still has the same magical properties and will try to reevaluate its form when full again (this time, it can get closer to the correct size - If the character uses a greatsword, it will become a shortsword, and then a longsword, and on the third try a greatsword). Only ranged weapons that can pierce organs may have the Natural 20 effect, however.
Bows are also special. The Spine can't duplicate non-metallic materials, and it doesn't understand guns and will ignore them, but complication is no limit. Thus, it will string itself with a tight metal "string" like the fat strings on an electric guitar. It can also form the various parts needed to make a crossbow. Arrows are a problem, though. When made into a bow it will also form one arrow. This is the only arrow that will have any of the above effects, although it can shoot normal arrows normally. The arrow will never break, and so it can be retrieved. Keeping a part of itself separate strains the ex-Spine, however, and if the arrow isn't brought back within physical contact of the bow for a day, the bow will start going mad and release a 1HP shock the next time it's touched. It will also quietly sob, non-stop, making sneak rolls impossible. It will do this until empty or the arrow is returned, and then it will "die," turning into the thighbone of a child and a small, mummified brain.