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.
Best Answer
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:
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.