"there is no weapon more deadly than the will" - What "the will" is ?
"Where there's a will, there's a way" also refers to "the will."
It means that you can make a choice but do so without really having the will to pursue it.
E.g. you know you have to clean your room so you make the choice to do so, but then you don't really have the will to do it.
But then you find out your friend is coming to visit, so you suddenly gain the will to clean up so your friend won't be offended by your dirty domicile.
Sometimes people talk about "the will to live," meaning a person who is sick or injured will persevere instead of surrendering to death because they have "the will" to do so.
Will has to do with motivation, though motivation can be something external motivating you, whereas willpower is something that comes from within yourself.