I was wondering if i could discard the location Narrow Sea(Core) to reduce the cost of the next stark character I play by 2, and use the event They shall not cross(LoW) to save it from being discarded. If i do this will the Shall not cross prevent the narrow sea's ability from being triggered. Or will all abilities resolve preventing me from losing the narrow sea and attaching TSNC as a boon. This may be a dumb question, but i am new to the game and still getting used to the mechanics of it. Thanks for any help on this.

There are two different answers here. One is pretty easy, the other is kind of technical.

1. Easy

In order to pay the cost of Narrow Sea's "reduce by 2" effect, the location has to be discarded. If something is saved from being discarded, it is treated as if it had never beed discarded in the first place. Since the location "had never beed discarded," you never paid the cost. And if you never paid the cost, you don't get the effect.

2. Technical

Saves are effects that interrupt the cost and the effect of whatever card/rule effect is initiating. So, the time to save something is after the cost has been paid, but before the effects are resolved. That's why costs still end up being completely paid, even if the target is saved from the effects. Well, when you discard Narrow Sea, you are paying the cost, not resolving the effect. So that cost is completely paid (ie, the location is completely discarded) before you ever get a chance to play the save. In short, the timing structure of the game really doesn't allow you to save something that is killed, discarded, or otherwise removed from play as part of paying a cost (even though many cards will specifically say "cannot be saved" after the cost; they don't really need to).