The planar ally spell beseeches an otherworldly power to send you aid in the form of a celestial, elemental, or fiend. In my case, the players are trying to summon a somewhat homebrewed fiend; an existing demon that I upgraded with a few extra abilities stolen from other monsters. Most importantly, this fiend has the limited magic immunity feature that I "borrowed" from the Rakshasa.
The Rakshasa's Limited Magic Immunity trait states:
The rakshasa can't be affected or detected by spells of 6th level or lower unless it wishes to be. It has advantage on saving throws against all other spells and magical effects.
My creature shares that same ability. It is unaffected by the spells of 6th level or lower unless it wishes to be, and has advantage on all other spells and magical effects.
The planar ally spell states:
After the creature completes the task, or when the agreed-upon duration of service expires, the creature returns to its home plane after reporting back to you, if appropriate to the task and if possible.
The creature the players are trying to summon has no other way to enter the material plane other than being summoned. So, it would very much prefer to remain on that plane as long as possible.
Since planar ally is a 6th level spell, this creature could be immune to the spell if it so chooses. If it did appear and makes an appropriate deal with the spellcaster (as per the planar ally requirements) but then didn't want to return back to its home plane after completing the assigned task, could the creature enact its limited magic immunity power in order to refuse to return home?
Best Answer
No, because the Creature has been sent by a third party entity, not the spell.
The Rakshasa itself is not being affected by the spell, but is a result of the spell.
The spell itself, as written, is communication with a 3rd party that sends you a representative. So the question to be answered is HOW the entity sent you the Representative. Gate (9th level) is likely the most efficient method for a deity or sub-deity to get his representative to your party. Is the Creature willing to defy the entity in question by not returning home (breaking the deal), or perhaps, after completing the contract, he was ordered to stay? The entity in question is undoubtedly powerful enough to re-send the creature right back to the party if the DM rules that the spell returns them home, or brute force recall the creature if they chose to stay.
But as RAW, the 6th level spell itself did not summon the ally, but rather made contact with an entity that sent the ally though unspecified means. So its up to the DM to determine if the spell / method used to send and recall the creature could be resisted. Deity powers are usually above 6th level.