Not really a Magic Item but maybe a Wondrous or advanced mechanical
(based on Randus Duthain, played by Brian Bergdall on the Critical Hit Podcast)
Exilarchy of Cogs Battlefist
A EoC Battlefist is a mechanical prosthetic made by the sentient Construct Race of the Cogs (or by some genius Artificer studiyng their technology). Actually supposed to be used as a swappable weapon for the Cogs warrior it can be reconfigured to connect to a living humanoid. Due to the intelligent design of the Exilarchy the Battlefist reconfigures itself to the needs of the user and can become much more then just a strong arm.
A EoC Battlefist has 2 Weight and is Clumsy
When you connect a EoC Battlefist to your living body, roll + CON as it analyses your bodily health and capabilities.
On a 10+ choose 2, on a 7-9 choose 1
* The EoC Battlefist looses the Clumsy Tag
* It configures a stronger outercasting, functioning like a shield. Take +1 armor
* If your Class Damage is lower then 1d8 upgrade it 1 step
* It has room to imbed a small crossbow (1 weight, near, can only store 1 ammo)
* It reconfigures the fingers to include different lockpicks, looking glasses and other thiefy tools