Here's the situation:
An ally is being restrained, and grappled by a NPC. The NPC is dragging the character.
My Four Elements monk character, seeing this, decides to use the Water Whip on the ally. He's going to take some damage, but since he's restrained, he will have disadvantage on, and likely fail the Dexterity saving throw and I will be able to retrieve him.
The Way of the Four Elements monk's Water Whip elemental discipline (PHB, p. 81) states:
You can spend 2 ki points as an action to create a whip of water that shoves and pulls a creature to unbalance it. A creature that you can see that is within 30 feet of you must make a Dexterity saving throw. On a failed save, the creature takes 3d10 bludgeoning damage, plus an extra 1d10 bludgeoning damage for each additional ki point you spend, and you can either knock it prone or pull it up to 25 feet closer to you. On a successful save, the creature takes half as much damage, and you don’t pull it or knock it prone.
Since this situation is somewhat different, my DM ruled that there would be a "tug of war" that took place instead of the typical Dexterity saving throw. The NPC dragging the character would instead make a Strength saving throw against my Ki Save DC. Essentially turning my Water Whip into a lasso.
His logic was that you couldn't use Water Whip on a creature restrained, and chained, to a wall and expect it to work.
So, essentially my question is:
Is it reasonable to make Water Whip a tug of war, requiring a Strength saving throw against my character's Ki Save DC, in situations where a target is being restrained?
Best Answer
It's definitely against the rules, but not unreasonable
The scenario as presented is fairly reasonable, but the ruling isn't RAW, and if this irritated you it's completely justified.
Plainly put, forced movement breaks grapples, and it doesn't care which creature is moved.
PHB pg. 290 under conditions, Grappled:
On top of that, Water Whip is a Dexterity save, not a Strength save. It also doesn't have an escape DC, which is telling. Like Thunderwave or Thorn Whip, when something gets moved by it, the magic is what's doing the moving and the only way to avoid that is to pass your Dex save and dodge it, or to use magic to counter it.
So in this scenario, your Water Whip should have torn the restrained party member from the grasp of whatever was dragging him. It doesn't matter that he's grappling the target. If the DM didn't want the thing to let go, the DM should have had it get dragged along with the restrained character. As a DM myself, when I do play a character and I'm escorting something I've restrained, I make it a point to tie myself to the creature I've restrained specifically to prevent scenarios like this from ever happening. I also keep a tether on my weapon for the same reason.
As for this:
This is actually bad logic. This is called a false equivocation fallacy, where the scenarios are not discussing the same thing. Walls and chains are unyielding, inanimate objects anchoring the target because they're fixed.