Does forced movement on a creature suffering the Grappled condition break the grapple?

If someone is suffering the Grappled condition and is then moved by another means what happens to the person providing the Grappled effect?

In a situation the fighter had grappled with the scary bird monster, another character who was "behind" the fighter casts Lightning Lure. The creature failed its Strength save and so was pulled towards the caster. We allowed that the fighter would pivot to allow the Lightning Lure to pull the creature.

Was this a fair reading of the rules? As Lightning Lure only has a range of 15' it seemed that (if using squares) that it would go from:

| M | F |   |   | C |


to

|   | F | M |   | C |


(Key: Monster, Fighter, Caster.)

I don't think there are additional considerations for Thorn Whip's longer range, but answers could include any such details.

The rules on Grappled condition seem to answer the first part of your question :

Grappled
[...]

• The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell. (Player's Handbook, p. 290)

So that's what happens to the person providing the grappled effect: he loses it's grapple if the creature is taken out of his reach.

As for the second part, here's what Lightning Lure does :

You create a lash of lightning energy that strikes at one creature of your choice that you can see within range. The target must succeed on a Strength saving throw or be pulled up to 10 feet in a straight line toward you and then take 1d8 lightning damage if it is within 5 feet of you. (Sword Coast Adventurer's Guide, p. 143)

Unless I'm wrong with square count, your caster seems too far away to reach the monster (space 1 = 5' from him, space 2 = 10', fighter = 15' and monster = 20'), which is over the cantrip's range.

Now if you placed the caster 5 feet closer, the spell would work. It would hurl the creature over the fighter and 10' (or 2 squares) towards the caster, but still in reach of the grappler, thus not ending the grappled condition in the process.

On a grid, the positions would change from [monster|fighter|space|caster] to [space|fighter|monster|caster].

From appendix A of the PHB, under Grappled:

• The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.

If the spell moves the grappled creature out of reach, they're no longer grappled. In your case, you mention 15 ft, that would probably move it out of the grappler's reach (typically 5 ft).

• "In your case, you mention 15 ft, that would probably move it out of the grappler's reach". That's what I first thought, before realizing the caster had to be closer for the spell to work. – Meta4ic Apr 1 '16 at 20:39