Can tremorsense be used to target already-cast spells?

Question

Specifically spiritual weapon and flaming sphere.

The scenario is a (10th-level or higher) Circle of the Moon druid casting a spell, then using Elemental Wild Shape to transform into an earth elemental and using its Earth Glide trait to completely burrow into the ground. They then use tremorsense to control the spell, attacking enemies with no chance of counterattack.

Can they use spells at all? Should the targets of the spells get any benefit?

I am thinking of making the spiritual weapon attack at disadvantage because the caster can't see the target. I figure that tremorsense lets you know where a foe is standing but doesn't let you see their body movements, so you are effectively attacking blindly.

Answer 1

It depends on the spell.

Here's how Tremorsense works, according to the Monster Manual:

A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.

Notice that Tremorsense doesn't give you to the ability to see. It lets you know accurately where vibrations are coming from, when things move or make a sound against material that you're in contact with.

Since it doesn't let you see, you would not be able to use a spell like Call Lightning while fully embedded in the earth. The spell requires you to "choose a point you can see within range", and you cannot see anything.

But spells like Flaming Sphere or Spiritual Weapon would work just fine, since they don't have that requirement. Flaming Sphere says that "you can move the sphere up to 30 feet" as a bonus action. Spiritual Weapon has similar wording. Since these spells don't require you to be able to see, you should be able to use them while relying purely on Tremorsense.

However, the enemy may figure out that they can just stand perfectly still to avoid giving away their location, or distract you by throwing things onto the ground.

Note that you make attacks with Spiritual Weapon at disadvantage if you're relying purely on Tremorsense. According to the Player's Handbook ("Unseen Attackers and Targets"):

When you attack a target that you can't see, you have disadvantage on the attack roll.

Answer 2

Absolutely, but it might not work exactly how you expected

Tremorsense can absolutely be used as you described. As tremorsense is defined (MM, p. 9, bold added)

...tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance. Tremorsense can't be used to detect flying or incorporeal creatures.

Tremorsense is usually used by burrowing creatures, enabling them to come up, attack their prey, and dive burrow back down. As such, the text above doesn't clearly define whether a creature can "see" a target through tremorsense alone. Your reading that "tremorsense lets you know where a foe is standing but doesn't let you see their body movements, so you are effectively attacking blindly" is completely viable.

Would attacks get disadvantage? Probably not

However, note that the rules for Unseen Attackers And Targets (PHB, p. 194-195) state both of the following:

When you attack a target that you can’t see, you have disadvantage on the attack roll.

and

When a creature can’t see you, you have advantage on attack rolls against it.

And even though your target can see the spiritual weapon, Jeremy Crawford has confirmed that the spellcaster is the one doing the attacking. So rules for unseen attackers apply here.

So if a druid Wild Shaped into an Earth elemental (having previously cast Spiritual Weapon), then dove into the earth via Earth Glide, and attacked a target they could pinpoint using tremorsense, the target would be "unseen" to them, and they "unseen" to the target.

And remember (PHB, p. 173):

If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20.

Thus, the roll would most likely be made normally, without advantage or disadvantage (this would not influence Flaming Sphere, since that spell does not involve an attack roll).

How powerful is this strategy?

This is a sound and clever use of tactics on the part of the Druid. But it's not insurmountable. The Druid can't cast spells while Wild Shaped (unless they are 18th Level or higher), so they can only maintain a spell, not renew it if needed. And as both of these spells are second level, a simple Dispell Magic spell will handily end them (unless they are upcast).

Keep in mind, a lot of tactics in DnD (or real life) rely on attacking while an enemy cannot counterattack. That's the whole idea behind most ranged builds. But that doesn't make these strategies all powerful. It will work, until it doesn't.