Can a planar ally use Limited Magic Immunity to avoid returning home?

Question

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?

Answer 1

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.

You beseech an otherworldly entity for aid... That entity sends a celestial, an elemental, or a fiend loyal to it to aid you, making the creature appear in an unoccupied space within range.

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.

Answer 2

I don't think so. Using the sage advice compendium's guidance on when dispel magic would work:

Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend. Here’s why: the effects of an instantaneous spell are brought into being by magic, but the effects aren’t sustained by magic (see PH, 203). The magic flares for a split second and then vanishes.

and this sage advice from Jeremy talking about how magic immunity works against summoned creature he seems to imply that if the effect is undispellable limited magic immunity wouldn't affect it:

Because of the Limited Magic Immunity trait, a rakshasa could stroll through the wall created by the wall of stone spell, but not if the wall becomes permanent and nonmagical (aka undispellable).

I think the creature with limited magic immunity would have to choose at time of casting of planar ally. After the spell has been cast there is no spell for the creature to use its immunity on.

Answer 3

As per the D&D 5e compendium:

When the creature appears, it is under no Compulsion to behave in any particular way. You can ask the creature to perform a service in exchange for payment, but it isn't obliged to do so.

The summoned creature is an ally, not a slave. They must be convinced to help you, often in the form of payment (in gold or other alternatives, as further detailed in the compendium).

The GM can adjust these payments based on the circumstances under which you cast the spell. If the task is aligned with the creature’s ethos, the payment might be halved or even waived.

So, given a prior relationship and alignment, you might agree that the creature may forgo payment.

A creature enlisted to join your group counts as a member of it, receiving a full share of experience points awarded.

This explicitly suggests that one of the "tasks" for said creature might to be to join your party.

Combined, if said creature is keen on remaining, they would likely negotiate for a deal that both your adventurers would find hard to say no to and one that is specifically worded to keep them on the material plane for as long as possible.

Summon spells usually end when they have succeeded, e.g. the item or creature that has been summoned has arrived. There is no magic in compelling them to remain, instead you must contend with their own effects, volition, and other priorities. As suggested by some of the other answers, most creatures probably prefer home or are bound to a greater entity that would not tolerate unjustified absences.