Michael Hopcroft
Mongoose
I'm not sure of the cost, but there ought to be a Silent Kill legendary ability for skulker types. This enables you to sneak up behind someone (like a sentry) and kill him without him uttering a sound by such means as strangulation, throat-slitting or a quick thrust to the heart.
I was thinking the cost would be roughly similar to Decapitating strike, because the essential effect would be the same (an instant kill). But it would only work if the target is completely unaware of your presence. if he spots, hears or detects you in any way (including by magic) you'll have to do him in the hard way, and there's a good chance he'll get some sort of signal out to his buddies before you can.
I was thinking the cost would be roughly similar to Decapitating strike, because the essential effect would be the same (an instant kill). But it would only work if the target is completely unaware of your presence. if he spots, hears or detects you in any way (including by magic) you'll have to do him in the hard way, and there's a good chance he'll get some sort of signal out to his buddies before you can.