Assume I use summon greater demon (Xanathar's Guide to Everything, p. 166) and choose a demon with the Magic Resistance trait.
Does Magic Resistance grant the demon advantage on the Charisma saving throw at the end of its turn against the summon greater demon spell?
Magic resistance on any creature description I can find in the Monster Manual states the following:
Magic Resistance. The Insert monster name here has advantage on saving throws against spells and other magical effects.
Your summon greater demon spell being part of a spell is, indeed, a magical effect holding the demon, so he would certainly have advantage against resisting its hold.
Summon greater demon however, has this in its description:
At the end of each of the demons turns, it makes a Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name.
The rules of advantage and disadvantage in the Player's Handbook state:
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and roll one d20. This is true even if multiple circumstances impose and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage.
Magic Resistance says:
... has advantage on saving throws against spells and other magical effects.
The control from summon greater demon necessitates a saving throw and is part of the spell description. It is therefore covered by Magic Resistance.
The only way to get around this would be to cause the demon to have disadvantage (therefore cancelling out any advantage). One such way is by knowing the demon's true name.
Yes - RaW; it’s a magical effect causing a savings throw.
I've gone through and listed the viable targets up to lvl 9th spell @ CR 10 or below.
Much greater risk with summoning a demon with magic resistance, especially if you choose not to do the circle.
Valid target without Magic Resistance:
Valid target with Magic Resistance:
*I count as invalid targets, being they are not natural to the Abyssal plane and already a corrupted humanoid.
*Draegloth - A draegloth is a half-drow, half-glabrezu demon, born of a drow high priestess in an unholy, dangerous ritual.
*Tanarukk - The demon lord Baphomet gladly shares the secret of creating tanarukks with those who entreat him for power. The process corrupts an unborn orc of the tribe, transforming it at birth into a creature much more savage than an orc.
The lack of cohesion between the lesser and greater is off putting. Lesser states "hostile to all" yet Greater states nothing.
