Can you maintain concentration on a spell from another plane?

Question

Here are two scenarios:

1. A powerful enemy caster is concentrating on some high-level offensive spell (Incendiary Cloud, Earthquake, etc.)--one PC casts Banishment and the caster fails the save. Can the enemy caster maintain the spell from the demi-plane?
2. A PC casts Expeditious Retreat and Blink on themselves: when transported to the Ethereal Plane, can they maintain the effect?

Currently, I'm leaning towards 'no' in the first case and 'yes' in the second, based on this last line from Blink:

You can only affect and be affected by other creatures on the Ethereal Plane.

In other words, as long as you're in the same plane as the effect you want to maintain, then yes.

Answer 1

There's a Sage Advice answer on this:

If you’re concentrating on a spell, do you need to maintain line of sight with the spell’s target or the spell’s effect?

You don’t need to be within line of sight or within range to maintain concentration on a spell, unless a spell’s description or other game feature says otherwise.

So, you can still concentrate on a spell, even if you or the affected target moves to another plane, unless the spell description specifically says that you can't. As long as you were in range, with line of sight, line of effect, or whatever the spell requires at the moment you cast it, it doesn't actually matter what happens afterwards.

As Tophandour notes, however, the DM may rule that you need to make a Constitution save to maintain concentration through the plane shift. Previous editions have stated that teleportation and planar travel spells impose various effects as a result of the disorientation and shock, so I think it'd be fair to say that it might disturb your concentration on a spell.

Answer 2

While in general I agree with anaximander's answer that nothing prevents a character from maintaining concentration from another plane in general, the specific case presented in scenario #1 is different.

The concentration rules state (PHB 203):

The following factors can break concentration: [...] Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.

Part of the description of the banishment spell says (PHB 217):

If the target is native to the plane of existence you're on, you banish the target to a harmless demiplane. While there, the target is incapacitated.

Thus, the enemy caster will lose concentration by getting incapacitated, unless he is not native to the plane they are currently on. This effect is explicitly stated in the spell description and is unrelated to him being on another (demi)plane, though.

Answer 3

There is nothing in the rules or the Banishment Spell right now to indicate that one must maintain some kind of line of effect to a spell's location in order to maintain concentration.

One might be able to argue that the incapacitated or killed clause in the concentration mechanic applies as you are no longer capacitated in your current plane, but the utter silence of the rules leaves it squarely in the hands of the DM (though you could effectively argue that because the rules don't say anything about it, it's not a concern, I think it's just a situation that's not covered and should be adjudicated).

As as DM, I think that your two points are squarely on target, all spells that affect a different plane should end when you are transported. But spell effects that affect you or creatures transported with you should continue though I feel like a Concentration save would be very much in order (DC 10).