The demon does not usually have the aura of the paladin …
We can reference this question on the definition of "hostile", which quotes the DMG p.244:
- A friendly creature wants to help the adventurers and wants them to succeed.
- An indifferent creature could help or hinder the group, depending on what the creature considers most beneficial.
- A hostile creature opposes adventurers and their goals but does not necessarily attack them on sight.
Making no assumptions based on the D&D tradition, which is not inherent in fate, the formulation of Summon Greater Demons in fact makes no provision as to whether the demon invoked is friendly or hostile – compare to Summon Little Demons (XGE p.167), which declares
Demons are hostile to all creatures
or other conjuring spells, such as Summon animals, Conjure Elemental, Celestial conjure, Conjure Fey, Summon Minor Elementals, Conjure woodland beings (PHB p.225-226), which all explicitly state that the creature (s) summoned are / are
friendly to you and your companions for the duration.
Because a spell does what it says, and nothing more, it cannot be assumed that the demon summoned by Summon Greater Demon is friend and therefore does not automatically benefit from the aura of the paladin.
… unless the demon is somehow already friends with the Paladin …
Nor does fate provide that the demon is necessarily hostile. It may be possible to summon a friendly demon, but that demon will not automatically be friendly as a result of the spell.
For example, if Paladin McLawfulGood had inexplicably befriended Shadowy the Shadow Demon, and Shadowy was by chance (or by DM fiat) the specific demon summoned by the spell, then according to DMG guidelines above, Shadowy would benefit from the aura of the Paladin.
Note that the spell does not give the caster any opportunity to choose the individual demon that is summoned, but allows the possibility that the summoned demon is known, as it includes a small advantage.
if you say his real name.
… but maybe only until the demon breaks free
By a strictly raw reading of the spell, when the demon is released, it
spends his turns chasing and attacking the closest non-demons to the best of his ability.
It is not an optional clause; therefore, a friendly demon who manages to break control would then be forced to attack the nearest non-demons. If this includes the Paladin or his allies, this may move the demon directly into the hostile category, and therefore not benefit from the aura.
That said, there are East a lot of ambiguity in this spell (as if the demon is forced to attack, or it just describes the behavior of more demons), so a reasonable interpretation will ultimately depend on the DM in this slightly absurd scenario.