Have you ever read 'The way of kings'? There are some really neat swords in this that do that. They are called shard blades, I won't spoil what they are, but I'll tell you how they work. First of all they are all (almost all) massive six foot long swords but weigh only a bit more than say a long sword. Next they cut through anything (except shard plate) effortlessly. Anything not alive is cut perfectly, anything that is alive is not cut but instead dies instantly but this leaves behind an intact corpse. If a limb is cut, just the limb dies, but it is also not cut (a second cut would sever it). Now Once you have attuned to it you can dismiss it at will and if you are disarmed the same thing happens so it is very difficult to steal, as the wielder needs to will it not to disappear when he lets it go. Here's the disadvantage though when you want to summon a shard blade you must wait ten heartbeats for it to appear. You could easily accept the same restriction and I'd say that would be worth +2. This makes it very difficult to steal, but provides a huge disadvantage to you if you get disarmed. I'd say that it would take one full combat turn to summon, but you could work out the details with your gm.
