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.
Best Answer
It depends on the spell.
Here's how Tremorsense works, according to the Monster Manual:
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"):