so I am running the dragonlance shadow of the dragon queen quest for my group currently and have run into a situation on possible house ruling (on top of another house ruling I will get to). Currently the problem situation is with the wizard and druid and their AoE spells. Ran into an encounter where the wizard threw up an upcasted wall of fog to obscure the entire encounter then proceeded to want to drop fireballs into the wall of fog that the party was also stuck in as well as the druid wanting to drop a moonbeam in the wall of fog as well.

I know the new spell casting rules, on both fireball and moonbeam, no longer require the targeted point to have to be seen. I am foreseeing this becoming a trend in the campaign where they just throw up a wall of fog and blanket the area with precision AoE spells since technically you do know the location of enemies unless they are hidden via stealthing. Previously I had house ruled that moonbeam had to see the square/space (since we play on a grid map) to move the moonbeam the round after it was cast because the player had intentions of shapeshifting into a tiny spider and hiding in a crack while he moves moonbeam around and nukes everything from orbit (he is a circle of the moon druid so he can cast moonbeam in wild shape). The PHBs rules say for the target point cannot be behind total cover but it does not cover moving magical zones out of your line of sight/effect. My argument for house ruling was that the player could not possibly always know and memorize the exact layout of the field of combat to sit in a crack and direct the moonbeam around corner where he could not see.

I am always for my PCs stomping an encounter with ingenious and tactical thinking and always find it fun and interesting the stuff they come up with, but this seems to be a nasty combo that unless you have blindsight, truesight or tremorsense, it's just a free win the for the PC's. Obviously in a dungeon grind they have to be more conservative with their spell usage, but those large one off tough encounters that are meant to tax them to the limit seem trivialized by this.

I was contemplating using a combo of rules from PHB 2014 and 2024 and reinstituting the point of origin must be seen, but am not sure if that is the proper path to go down.

would love any comments or suggestion.

-Dispel Magic works on the Fog Cloud
-Strong winds work on the Fog Cloud
-Enemies can use the tactic against the players as well
-Some creatures have Blindsight or Tremorsense and can ignore the restrictions (and you can put Blindsight on a humanoid when it makes sense, maybe a single armored foe has it like a PC could take the Blind Fighting style at level 1)
-Creatures can Hide at any point while in (or behind) the Fog Cloud, meaning the players no longer know the locations of the enemies

Just poke holes in the plan here and there. Let it work sometimes because it's effective against small enemies, but against large enemies, it has flaws. Just give them some enemies that challenge the strategy here and there (not constantly) and make them try to guess if it's the most effective use of resources against each enemy.

It's not really a free win for the PCs -- the monsters know where the PCs are too, and can walk over and beat on them (without even having disadvantage on their attacks -- disadvantage for unable to see target is negated by advantage for being unseen). Most of the time it's actually more impairing for the PCs than the monsters, because a lot of spells (as well as the sculpt spells evoker feature) require vision.

Quote from Epicfayle25627 >>

[...] I know the new spell casting rules, on both fireball and moonbeam, no longer require the targeted point to have to be seen. [...]

I was contemplating using a combo of rules from PHB 2014 and 2024 and reinstituting the point of origin must be seen, but am not sure if that is the proper path to go down.

 AFAIK, the rules about targeting are essentially the same, aside from clearer wording. Depending on the spell, you may or may not need to see the target. Spells that say "that you can see" or similar wording require the caster to be able to see the target.

Some relevant rules:

A Clear Path to the Target. To target something with a spell, a caster must have a clear path to it, so it can’t be behind Total Cover.

Areas of Effect. Some spells, such as Thunderwave, cover an area called an area of effect, which is defined in the rules glossary. The area determines what the spell targets. The description of a spell specifies whether it has an area of effect, which is typically one of these shapes: Cone, Cube, Cylinder, Emanation, Line, or Sphere.

Area of Effect

[...] An area of effect has a point of origin, a location from which the effect’s energy erupts. The rules for each shape specify how to position its point of origin. If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect. To block a line, an obstruction must provide Total Cover. See also “Cover.”

If the creator of an area of effect places it at an unseen point and an obstruction—such as a wall—is between the creator and that point, the point of origin comes into being on the near side of the obstruction.