The Lightning Sword was the divine weapon of Perseus, he uses it to channel His lightning powers. The sword's compacted form is a small cylinder/baton which only at the touch of Perseus will glow and transform into its true sword form. It till now would revert into its dormant form unless he decided for it to stay in full form.
The sword is so powerful it banished Hades back where he belongs; the Underworld.
Now tainted with dark energon the sword is in its full form and if it was to get in to the hands of the Decipticons the Autobots to could end up in the Underworld along side Hades.
The Sword Blade Is 14.5cm Long