A creature is on the Material Plane and can perceive someone on the Ethereal Plane using the True Seeing spell. Can that creature cast a spell that only requires that they see the character on the creature in the Ethereal Plane?

For example, could someone on the Material Plane that has truesight cast Eyebite on a creature it can see in the Ethereal Plane? Eyebite just requires that you see your target, and with truesight the caster can see the creature on the Ethereal Plane.

Etherealness says:

Creatures that aren't on the Ethereal Plane can't perceive you and can't interact with you, unless a special ability or magic has given them the ability to do so.

True seeing gives the caster the ability to perceive the person on the Ethereal Plane. The text for Eyebite implies it only requires you to see the target. So can the person on the Material Plane with True Seeing active cast Eyebite on someone the see in the Ethereal plane?

I'm using Eyebite as an example, but it could be applied to any spell that only requires sight.

No, not unless a spell specifically allow it to cross planar boundaries

Etherealness says:

You ignore all objects and effects that aren't on the Ethereal Plane

Unless the caster has a means to transport their effect to the Ethereal Plane, all spell effects that they create will be in the Material Plane. And since a creature under the effects of etherealness is immune to effects not in the Ethereal Plane they would not be affected by them.

Additional support comes from the part you already quoted:

Creatures that aren't on the Ethereal plane can't perceive you and can't interact with you, unless a special ability or magic has given them the ability to do so

True seeing might give a creature the ability to see you, but it does not magically give them the ability to interact with you. You are still on a separate plane. Thus, they are not able to target you with effects for this reason as well as it is a type of interaction.

Spell which target by sight still have to meet other requirements

As a related aside: when a spell says that you must be able to see the target, it does not mean that this is the only requirement that the spell has for being able to target something. All spells, for example, must have a clear path to the target (unless something in the spell effect says otherwise) by default.

Though nothing says it explicitly in the rules, it seems the most logical ruling would be that you do not have a clear path to things on a separate plane from yourself. Spell effects cannot cross planar boundaries unless they explicitly say so. Adding a requirement that you must be able to see the target does not bypass these other considerations.

Yes

While "[y]ou ignore all objects and effects that aren't on the Ethereal Plane", where a particular spell's effect happens depends on the target of the spell.

For example:

• Fireball: "A bright streak flashes from your pointing finger to a point you choose within range ...". Your finger is on the material plane, therefore, so is the Fireball.




• Hold Person: "Choose a humanoid that you can see within range." You can see them, they are in range (since distance on the Ethereal and the Material have a 1:1 correspondence), the spell can affect them regardless of which plane they are on.




• Suggestion: "a creature you can see within range that can hear and understand you." If they can hear material things then it affects them, if they can't, it won't. We know from the Etherealness spell that material creatures can't "perceive" ethereal creatures without magic and True Seeing only covers sight not hearing. However, while we know ethereal creatures can see 60 feet into the material we don't know if they can hear into it - in the Forgotten Realms the answer appears no be no but your DM may rule yes.