Is this random area of effect spell deviation house rule broken?

Whenever an AOE spell is cast, the actual target center (for circles) or target angle (for cones) differs from the intended target by a factor of 5 feet/degrees * spell level / caster level. Direction of deviation is determined by throwing 1d8 with 1 representing “north or up” and the clockwise each subsequent number representing a quarter change (1d2 for cones left or right)

Explanation

My players and I use Fantasy Grounds for our online role playing.

Up until now, whenever there’s an area of effect spell (spike growth, fireball, etc) I simply draw a circle or whatever center at the point they indicate and we use that to track the AOE of the spell. That’s all fine but due to the mechanics, it produces a couple of undesirable consequences:

1. They get laser like precision, being able to cast spells that affect the maximum number of enemies and never hit an ally.
2. They are aware of where effects start so they can avoid the affected areas while enemies don’t.

For the second point we’ve agreed to simply not show them where the exact area of effect is, I will see it but they won’t. That improves it but they can arguably still keep it in mind with precision by simply counting squares on the map.

We have, therefore, agreed to introduce some randomness in the “aiming” of the spell. You intent to put the center somewhere but, since you’re just human after all, you don’t get it perfect.

Current line of thought is to make it small but noticeable, maybe up to 2.5 to 5 feet away from your intended target. That’s easy to make it random but there have been chats for making it rely on spell caster level vs target ability or vs spell level (I.e. distance from intended target is 5 feet * spell level / caster level).

Example

You are a level 5 wizard casting a fireball spell. You select a target center, and roll 1d8 getting 5. The fireball impacts at a point roughly 3 feet below your intended location.

Note

After seeing the linked question, I still prefer this to a check against an AC. Isn’t this system no matter how good you are you’ll never get it exactly right (which is how good aim works in real life). The better you are, the closer you get, but a level 15 wizard will still get it wrong by a feet.

• Commented Jan 21, 2020 at 0:03
• In what way are you asking if the change is broken? Clearly you know that it nerfs AoEs, since that is the intention. Are you asking if this is abusable? If it is imbalanced? Or if it is a good change? Would this question be better served by simply trying it out? Commented Jan 21, 2020 at 0:49
• You use "we" through your answer, is this a plan come up with by everyone and you're happy but still asking for a third party to take a look? Commented Jan 21, 2020 at 1:09
• Yes, we are somewhat inexperienced modifying the game so I’m asking whether it could break the dynamics of AOE or have unintended consequences Commented Jan 21, 2020 at 7:05

Playtest it

The rule is not broken in the sense that it meets the definition of an algorithm that I was taught as part of my computer science degree: its a procedure that is followable and always returns a result.

Whether it results in the play experience that you are looking for is not something you can tell without trying it.

Issue

We have, therefore, agreed to introduce some randomness in the “aiming” of the spell. You intend to put the center somewhere but, since you’re just human after all, you don’t get it perfect.

Analysis of the consequences:

1. They get laser-like precision, being able to cast spells that affect the maximum number of enemies and never hit an ally.

2. They are aware of where effects start so they can avoid the affected areas while enemies don’t.

1. Yes, this eventually happens.
2. Yes, knowing the place they affect, they can avoid the area. This is expected. But, the spell sometimes is too fast for them to move.

The problem is to add randomness where there isn't and to weaken the magic characters, i.e nerf them. They are supposed to have these skills cause they are already nerfed on life points, physical strength and number of spells. So, the less accurate they are, the higher the probability they will die on combat.

That being said, your point makes sense and it sounds reasonable. So, I would play on your table without a problem.

Meta-knowledge interfering on the Game

It's important to limit the meta-game influence on the players' actions. It seems that meta-knowledge of the game could be affecting the two described consequences that you pointed out. Example, the Sorcerer Player just cast a fireball spell, the character didn't say anything to the group, why would the other characters know the area the spell would affect?

• +1. Yes, pinpoint accuracy fireballs are weird, but then so is an archer who shoots an arrow through the spaces of two of his friends to try and shoot a guy who's grappling a third friend. It's a game and there are balance concerns with changes like this. Commented Jan 21, 2020 at 8:12
• However I usually apply disadvantage on the situation you're describing and treat critical failures as a hit to your own. Plus hiting with an arrow is a lot less precise. As long as you hit A part of the target you make damage, it doesn't really matter if you hit leg, arm or torso so it is assumed there's deviation whereas with fireball (or other AOE spells) it's like magical perfect reach: "put it in this exact point so that it hits all this enemies but none of the allies by a couple of inches" Commented Jan 21, 2020 at 15:17
• Yes, the Golden rules apply, as well as house rules. However, this might unbalance the game indeed. Commented Jan 21, 2020 at 15:56