What happens when Blade Barrier overlaps Wall of Force?

Question

In a game I was DMing, a player created a wall of force to give himself and allies a safe place to fall back behind. After combat commenced, a blade barrier spell ended up being cast as a ringed wall. After the circle was cast, the cleric casting it drew the line on the map, and it was then noted that doing it how he wanted to would intersect with the wall of force.

Reading the two spells, I could find no obvious reason why blade barrier couldn't be cast in the same area a wall of force was in. Unlike wall of force, blade barrier has no restrictions about where it can be cast.

While blade barrier does state that it effects creatures, and doesn't technically say that it damages objects, I have made a house rule in my campaigns that any area damage effect is assumed to damage objects as well as creatures in its area of effect, unless the spell itself states specifically that it does not effect objects.

So, I ended up saying that the wall just took the damage for being in the blade barrier's area, sans 30 points each round for hardness. Is there some rule I've missed that would dictate a different result? Is there any good, non-rule based reason why I should not allow these spells to interact this way?

Answer 1

I think you made the correct ruling, but if I was inclined to disagree, I might point to this:

From Wall of Force:

The wall must be continuous and unbroken when formed. If its surface is broken by any object or creature, the spell fails.

This implies the wall, made of force, can't be formed when there's something in the way of it. If you read what the blade barrier is made of it says:

...blades shaped of pure force springs into existence...

You could argue the blades of force can't be formed in the solid wall of force and simply wouldn't affect that area.

If you think the blades can form, you'd have to determine the axis of the whirling blades. Do the blades whirl on their own axis but otherwise stay in a fixed point, or do they whirl around like a swarm of flies within the area of affect? Based on the spell description, I would argue the former:

An immobile, vertical curtain of whirling blades

If the blades spin about but otherwise stay in a fixed point, you could argue they would never gain the momentum to spin, like a sword stuck in stone. However, if the blades do swarm around the area of effect, damage would continue as normal.

I wouldn't fault a DM for ruling one way or the other. That is an interesting situation to adjudicate.

Answer 2

Imagine the wall of force effect as an actual wall

Most walls are solid barriers, and spells that create effects often can't create their effects through walls. Spell Failure says, "If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted."

The spell blade barrier creates an effect that, in this case, is "a ringed wall of whirling blades with a radius of up to 5 ft. per two levels [that's] 20 ft. high." Casting a blade barrier spell so that its effect overlaps with a low wall either causes the spell to fail or, more generously, causes the blade barrier effect to wrap over the low wall… not go through the wall.

That is, the blade barrier effect can't be shaped—in this case only its radius can be picked by the caster—so part of the blade barrier effect must be through the wall. Yet the blade barrier effect must stop at the wall and be on both sides of it. Thus the blade barrier effect is no longer contiguous, a necessity for the effect's successful creation. The GM may rule that the blade barrier effect conforms to the wall, wrapping around or over it (yet not through it), but a GM may instead rule that—because the characteristics of the spell can't be made to conform to the wall—the casting the fails, and the spell's wasted. In either case, though, the wall (or a wall of force effect) would remain unharmed by the blade barrier effect.

Essentially, what's being tried here is like trying to summon a Huge (tall) creature into a 15-ft.-by-15-ft. space that's split by a 10-ft.-high-yet-paper-thin wall: from several vantages the caster may have line of effect to all of the squares individually (see Aiming a Spell on Line of Effect), but the spell's overall effect can't be made to conform to the space into which the caster wants to put it. Trying to do that typically just ends up with the spell failing.

This is different from an area spell that's a burst or spread like the fireball spell (see Aiming a Spell on Area). The caster of the fireball spell picks a grid intersection as the spell effect's point of origin and counts squares from that point of origin to determine what's caught in the spell's area. This allows a fireball spell that's cast up in the air to affect stuff on both sides of a low-lying wall… and the wall.

A caster doesn't need a fireball spell's area to be clear of obstructions for it to be used—that's handled by the burst mechanics—, but a spell that creates an effect over an area requires more delicacy in its grid placement than a burst.