Upper_Krust
Legend
Hey ShadowofNarcissus amigo!
It wouldn't be automatic. But the other gods would see him as a threat to the status quo (depending on how active they were) and probably gang up on him to imprison him.
Its possible he could keep a low profile, perhaps maintain an Avatar to throw the gods off the scent. You would suspect as soon as he actually 'did something' of note that the gods would be aware of his presence - especially those that sit on the Throne of the Gods.
That's probably up to the DM and something to be roleplayed. However, why would deities of different alignments agree to it...?
They would likely only get involved if they attempted to extend their influence beyond the universe.
The dimensional guardians will intervene if he tries tampering with the dimensions.
At a certain measure of power the Gods would probably concede (they are not stupid). But of course there are other Sidereals to contend with as well as monsters that might be attracted to the 'scent' of an old one.
Imagine sidereals like fish, certain monsters (anomalies for instance) might prey on those 'fish', especially the active ones that wriggle about a lot.
Not all sidereals are imprisoned per se', but they are likely well hidden, perhaps operating interdimensionally. Once they try interfering in the affairs of immortals is when they are likely to run into trouble.
Cool.

If a greater deity somehow becomes an Old One is he automatically imprisoned or do the existing immortals try to imprison him?
It wouldn't be automatic. But the other gods would see him as a threat to the status quo (depending on how active they were) and probably gang up on him to imprison him.
Do they try to imprison him even if he goes out of his way to avoid upsetting the balance?
Its possible he could keep a low profile, perhaps maintain an Avatar to throw the gods off the scent. You would suspect as soon as he actually 'did something' of note that the gods would be aware of his presence - especially those that sit on the Throne of the Gods.
Could he make an agreement with the existing immortal pantheons to limit his involvement in their affairs and as a result remain free? ...at least until he oversteps the agreed upon boundaries?
That's probably up to the DM and something to be roleplayed. However, why would deities of different alignments agree to it...?
What if the existing immortal pantheons lacked the ability to imprison him and the other sidereals, even first ones tried to face off against him, but failed.
Would a Time Lord or Neutronium Golem get involved?
They would likely only get involved if they attempted to extend their influence beyond the universe.
The dimensional guardians will intervene if he tries tampering with the dimensions.
What if he defeated them, refused to ascend further and went into hiding? Would he be hunted?
At a certain measure of power the Gods would probably concede (they are not stupid). But of course there are other Sidereals to contend with as well as monsters that might be attracted to the 'scent' of an old one.
Imagine sidereals like fish, certain monsters (anomalies for instance) might prey on those 'fish', especially the active ones that wriggle about a lot.
Not all sidereals are imprisoned per se', but they are likely well hidden, perhaps operating interdimensionally. Once they try interfering in the affairs of immortals is when they are likely to run into trouble.
I ask because I have a greater deity (former hero-deity) who managed to become an Old One, has no interest in the affairs of other immortals, and spends most of his time adventuring in disguise. Mostly because his adventuring party (his closest friends) are all still hero deities and epic mortals.
He never lets on what he has become and goes on secret quests to further his understanding of what he has become. The only other being aware of what he is would be Charon, The Boatman of The Styx, and Charon, so far, seems to be ok with it as they're allies/family (Charon raised my character).
Cool.