I'm actually looking at creating a similar sword but it will probably have to be an Item of Power.

I'm mirroring my character off of Annja Creed from the Rogue Angel novels by Alex Archer. She's an Indiana Jones Type Archaeologist who travels the world on several adventures and in the first novel gets bonded to Joan of Arc's long lost sword from the 15th century.

This sword is just a plain sword but it exists in Otherwhere when not in use and Annja can call it instantly when she needs to use it.
It heals her a little faster but not unnaturally so. She may also be immortal now since a couple of other characters linked to the history of the sword have been living since Joan of Arc's death. It is thought by some that the Sword actually brings Annja to where she needs to be to help stop evil doers. 

I was actually thinking the easiest way to do it as an Item of Power was to reduce the One time Discount to +0 to account for the fact that most of the time the sword is in a pocket dimension (Otherwhere). Not sure if this is legal but since Items of Power have to get GM approval it's not that much of a stetch.

So as a Base Item of Power it would be
Discount [+0]
Wizard's Consitution [+0]
It's a Sword [Weapon:2]
Unbreakable
+ whatever other powers I wanted to add to keep it in the -1 , -2 refresh range.

Do you think that would work?