How do the Wall of Force and Wall of Light spells interact?

Question

In 5th edition D&D, suppose I cast a hemispherical wall of force, trapping a creature inside it on the ground. We will make the hemisphere 5 feet in diameter so it encompasses a Medium-size creature. There's no saving throw for this, based on my reading of wall of force.

Now suppose my esteemed friend and colleague casts wall of light inside my hemispheric wall of force. Wall of light can be 5 feet thick and at least 10 feet high so in theory could completely fill the hemisphere. Even if the creature inside makes the initial saving throw vs. wall of light, on subsequent rounds it will take 4d8 damage with no saving throw.

If both casters maintain concentration (10 min for both spells), whatever is inside has no chance to escape and will take 4d8 damage about 100 times barring teleportation, disrupting their concentration with a spell or ability, disintegrating the wall of force, some form of damage immunity, or I suppose burrowing speed.

Am I missing something? I ask because our party's lore bard and wizard are contemplating this combination. It seems quite powerful.

Answer 1

Wall of Force provides total cover against Wall of Light

The dome created by Wall of Force provides total cover for anything inside the dome from any effect outside it.¹ This means that the point of origin for Wall of Light must be outside the dome, as described in the spell targeting rules:

To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

With the point of origin outside, the spell's area cannot extend through the Wall of Force, as described in the rules for areas of effect:

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.

This means that if you first trap the target with Wall of Force, you will not be able to hurt them with Wall of Light afterward.

Try casting the spells in the opposite order, using a readied action

However, there is a way to get around this problem: make sure that Wall of Light is cast first, before Wall of Force. The complication is that you don't want the target to be able to move out of the light before you've trapped them with Wall of Force. So, if Lenny is casting Wall of Light and Fran is casting Wall of Force, then the proper setup is:

1. On Fran's turn, she casts Wall of Force and readies it to release with a trigger of "immediately after Lenny casts Wall of Light";
2. On his turn, Lenny casts Wall of Light on the target;
3. Fran's readied action triggers and she uses her reaction to release the Wall of Force, trapping the unfortunate target before they have time to move out of the way.

Alternatively, if Lenny is higher in the initiative order:

1. On Lenny's turn, he casts Wall of Light and readies it to release with a trigger of "when Fran gives the signal";
2. On Fran's turn, she starts by giving Lenny the signal (verbally, a free action);
3. Lenny releases his spell as a reaction;
4. Fran continues her turn and casts Wall of Force, trapping the victim.

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¹ See this answer, which establishes the basis for Wall of Force granting total cover.

Answer 2

You can't cast spells into the Wall of Force dome.

Once you've trapped the creature inside the dome, it has total cover. From the spellcasting rules: "To target something, you must have a clear path to it, so it can't be behind total cover."

Answer 3

It works as described

They would interact exactly as you described. It is a very powerfull combo that is entirely within the rules.

The only thing to note is that wall of light must be cast before wall of force, as you need a clear path to target of the spell.

You can also achieved the desired effect without the wall of force by using any means of immobilizing the enemy, although for making it nearly impossible to escape, the wall of force is perfect.

Great job finding this combo.