The spell call lightning has a part that reads:
If you are outdoors in stormy conditions when you cast this spell, the spell gives you control over the existing storm instead of creating a new one. Under such conditions, the spell’s damage increases by 1d10.
A player tried to use this line which I considered to be flavor text to declare that an opposing NPC couldn't use call lightning to attack him because he already had "control over the existing storm" and that the enemy would have to wrest control of the storm from him. (The DM allowed this as an Arcana contest.)
Does the phrase in the description of the call lightning spell actually have any meaning other than extra damage and flavor?
Best Answer
They only can't if 'control over the storm' is exclusive, which the text does not indicate in any way.
By a simple english reading, 'control over the storm' does not exclude others from also controlling the storm. Control is not an exclusive verb in english, although that's the more common usage.
In situations where a line like that implies an absolute control, it goes on to define the things you can do with your 'control over the storm', such as moving the clouds, water, etc. As it doesn't do that in this instance, while it is rules text, it does not preclude someone else from casting Call Lightning while under the influence of the same storm, neither does it allow the Call Lightning caster to move the clouds around, increase/lessen the rain, or anything else.
To further explain, 'control over' is only meaningful in DnD Rules-As-Written if what 'control over' does is meaningfully defined or has an extremely clear common-usage meaning (like 'swing a sword'). Since it isn't, you do have 'control over' but can't use it to do anything. If a different ability let you do to 'anything you magically control', say an ability meant to be used with Dominate Person and it's ilk, then you could use that ability and the line from Call Lightning together to do something (likely something silly). But as is, it doesn't actually allow you to do anything.
That specific situation, though, is the kind of thing that these games have a GM for - to apply sensible houserules to interesting or edge-case situations to help support the narrative structure of the game. Two storm-users struggling to control the storm and rain lightning on their foes is an extremely interesting thing to happen, moreso than simply casting a spell and dealing damage - and the player (intentionally or not) and the GM have added to the game by having that happen.