[RPG] Is it possible to combo the spells Create and Destroy Water, Shape Water, and Lightning Bolt in order to make LB more powerful


Asking on behalf of the group's Wizard.
He wants to electrocute the BBEG and have the spell deal extra damage through clever use of game mechanics. As a DM, I am all for it, as long as the rules somehow support it.

Create Water. You create up to 10 gallons of clean water within range in an open container. Alternatively, the water falls as rain in a 30-foot cube within range,* (PHB pg. 229)

Is this something you could cast, and either cast Lightning Bolt (8d6dmg) on the target being rained on(on your next turn / action), or use Shape Water to form a cube around the BBEG, and then Lightning Bolt to the face?

I personally think something being wet is conductive, which I might say means that the target being rained on would have disadvantage on the saving throw. My player really wants to try and argue that being wet yields a Damage Vulnerability (2x dmg), therefore causing lightning bolt to deal (8d6)*2 lightning damage.

Is there a rule I haven't found in the PHB about this already? I do really want the player to feel cool, especially since this upcoming week there is a BBEG confrontation and this might turn the tide for my group.

Best Answer

Imposing Vulnerabilities is not (strictly) in the DM's discretion...

Vulnerability is a very strong mechanic, and I would be hesitant to impose vulnerability unless it's written somewhere in the creature's stat block. It doubles damage received, and from the scenario you describe, Lightning Bolt with Vulnerability looks to deal about 56 damage on average for a use of one 1st-level spell (Create water) and one 3rd-level spell, all in two Actions. That is just too strong, taking out a third of an average CR5 creature's HP in one turn (not to mention you can do it again on the next turn, because the creature is still wet).

That mechanic is ripe for abuse (especially when you turn it around, and use it against the players) and I wouldn't recommend it.

... but Dis/Advantage is!

Instead of Vulnerability, give the creature Disadvantage on the Dexterity Saving Throw against Lightning Bolt. It is totally in the rules, and is balanced:

(DMG 239)

Advantage and Disadvantage

... Characters often gain advantage or disadvantage through the use of special abilities, actions, spells, or other features of their classes or background. In other cases, you decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result.

This way, the damage is still within sane levels, but the likelihood of the creature saving for half damage is reduced.