The situation is like this. PCs having a fight on a battlefield with several very deep chasms. One of the PCs attempt to grapple an enemy, then moves with him into one of the chasms (bringing himself and his target down and potentially very much dead). Looking through the rules, I have found only the following info about grapple and movement:
When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.
No other rules are preventing the PC from just moving into the chasm and dropping his enemy in it. Or at least I have found none. If I remember correctly, in previous editions there were some rules pointing out, that the victim in this situation is subject to some save or an ability check. That was my ruling during the game.
My question then is basically this:
Is there any rules about what options the grappled creature has in case the enemy tries to push it into dangerous terrain, such as a long fall from a high height?
I am looking for RAW answers, because I want to balance my judgments on at least some rule base (or certainty that there is none). My players are OK with my rulings, but I feel uncomfortable nerfing their strategies just because I feel it is wrong.