What happens when a caster loses concentration on a banished creature?

Question

The scenario is as follows.

The evil wizard summons forth a demon to fight in combat against the party with the summon greater demon spell, which failed two saves against the wizard. The party's sorcerer successfully uses the banishment spell on the summoned demon (a glabrezu in this case, as he cast the spell at 8th level). The evil wizard, feeling unable to successfully break the sorcerer's concentration, just drops concentration on summon greater demon and instead casts some other concentration spell (in my case he cast the above spell again, but that's not relevant right now).

If the sorcerer's concentration on banishment drops at any time before its duration ends, what happens to the glabrezu? Does it come back? Do rounds where it was not present count against the 1d6 extra rounds of presence (from dropping concentration on summon greater demon)?

I would also be interested to see answers if the original demon was uncontrolled (it had succeeded its save against the wizard), and if the protective circle was in place.

Answer 1

If the sorcerer drops his concentration on Banishment, the Glabrezu may return.

If the demon was under the evil wizard's control:

If the demon failed all of his Charisma saving throws, then the caster can end the spell. The demon is gone and will not return when Banishment ends.

If the demon was not under the evil wizard's control:

When the evil wizard drops his concentration on the initial Summon Greater Demon spell, you should roll a 1d6 to determine how many more rounds it will be around for. Start counting the rounds now.

If the sorcerer (who casts Banishment) drops their concentration, within those 1d6 rounds, the Glabrezu will return.

As per the Summon Greater Demon spell, it will be uncontrolled by the evil wizard.

The summoning circle

The circle of blood that the demon was summoned in has these properties:

As part of casting the spell, you can form a circle on the ground with the blood used as a material component. The circle is large enough to encompass your space. While the spell lasts, the summoned demon can’t cross the circle or harm it, and it can’t target anyone within it.

The spell states that the demon can't cross the circle or harm it, and the caster of the Summon Greater Demon spell resides inside.

Banishment interacts independently of the circle regardless. It banishes the target to a harmless demiplane or the targets' home plane. So I think it's reasonable to say that Banishment can be cast without any consideration to the circle.

Answer 2

It's not clear. There isn't a rule for resolving conflicts between two spells like this.

Normally, if the wizard drops concentration on purpose, the demon would disappear.

... the demon disappears when it drops to 0 hit points or when the spell ends.

If the demon broke free of the wizard's control, then it would get 1d6 rounds after the point when the wizard drops concentration.

If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn’t disappear for 1d6 rounds if it still has hit points.

Only an uncontrolled demon can stay after the spell expires.

Banishment says:

If the spell ends before 1 minute has passed, the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.

These two spells contradict each other. It's not clear what would happen.

It is a valid ruling that the summon spell is ended and the only spell affecting things is the Banishment spell, which clearly says "the target reappears". In that case, why would it be limited to 1d6 rounds? The spell that gave it that time limit is gone. Wouldn't it be here to stay?

Another valid ruling might be that nothing is keeping it here since the summon spell is gone. Maybe the creature flickers breifly into view when the sorcerer drops Banishment, but nothing mechanically happens.

There are no rules written for resolving this kind of conflict. Your DM needs to make a ruling. I suggest appealing to the highest law: The Rule of Cool. What's more dramatic? What's more fun?