We were fighting some berserkers. I was already flying (fly spell) so decided to use eldritch blast and the Grasp of Hadar invocation.
Once on each of your turns when you hit a creature with your eldritch
blast, you can move that creature in a straight line 10 feet closer to
you. (XGtE, p. 57)
As I flew over one of the Berserkers, I zapped him with EB twice (at 5th level this cantrip emits two blasts) and applied Grasp of Hadar on him. I pulled him toward me ten feet, straight up. Once I had pulled him up, he didn't stay up; the DM ruled that he had to fall since the Grasp doesn't grapple him nor hold him in place.
I recommended to the DM that he take an additional 1d6 falling damage; I rolled it and he agreed.
Is this correct, or did we misunderstand how Grasp of Hadar works?
Imbedded question if the answer to the above is "yes, d6 damage was correct" –
Something that had us scratching our heads was how far offset do I need to be before the fall won't be enough to cause damage? I can be as high as 90' up and EB will still reach, but if I am too far offset the vertical height of the Grasped creature won't be 10 feet (for example at a 45 degree angle they would be about 7 feet off the ground). For the time being the DM is going to wing it, but I thought I'd ask so that I have something more concrete to offer him.
A related question about thorn whip is here and I suspect that the Four Elements Monk Water Whip feature has a similar behavior.
Best Answer
Drop 'em like they're hot
This was a phenomenal maneuver! As you've said, the warlock invocation grasp of hadar states:
Since you are flying and above them, that does mean you pull them off the ground. There is no difference in the language or any requirement that they stay on the ground, so they don't!
And since you've pulled them 10' up, then they should fall as well, triggering the 1d6 bludgeoning.
A clever use of your abilities and invocation - well done!
Do you have to be directly be above?
As KorvinStarmast wrote in a comment, a DM ruling that you must be directly above them for this to work is reasonable, but depends on the rules being used for diagonal movement.
The PHB simplifies things with treating any grid movement as 5', even diagonals. There is an optional rule in Chapter 8 of the DMG where diagonals use more movement, but that's optional. Playing Theatre of the Mind and sticking with actual measurements would require being directly above the target in order to pull the 10' up to cause damage. However, those playing Theatre of the Mind could still use the diagonal rules from either the PHB or the DMG in addition to their imagination.
In some ways, it may be easier to say you need to be directly above, otherwise you may be under the 10' gate for falling damage. But talk with your DM about what system they want to use consistently.