I've been making small 2D games for about 3 years now (XNA and more recently LWJGL/Slick2D). My latest idea would involve some form of "talent tree" system in a real time game.

I've been wracking my brain but can't think of a structure to hold a talent. Something like

"Your melee attack is an instant kill if behind the target"

I'd like to come up with an abstract object rather than putting random conditionals into other methods. I've solved some relatively complex problems before but I don't even know where to begin with this one.

Any help would be appreciated - Java, pseudocode or general concepts are all great.