I am presently trying to imagine possible endings to a long-running campaign that I am GMing for wherein the main villain is a lich.
However, I had made some modifications to the phylactery to more closely resemble the popular trope often found in literature, namely the phylactery only being destructible through special processes such as dropping the ring into the fire from whence it came or destroying all seven of them through powerful magics. Ultimately, I suppose this results in the phylactery being slightly more difficult to destroy if certain conditions are met.
With that being said, in the event that my players will be unable to destroy the phylactery and thus kill the lich, they may need to settle for imprisonment instead.
Would an anti-magic field suffice for rendering the phylactery null and void, and therefore one could potentially serve as a suitable prison?
There are two reasonable ways to interpret this that I can think of.
The first is that the lich's phylactery is a kind of non-artifact magic item, and therefore placing it inside the antimagic field is sufficient to suppress the lich's rejuvenation.
The second is that the lich's rejuvenation is a Supernatural ability, and therefore keeping the lich's remains inside an antimagic field prevents it from rejuvenating.
The second approach is more solidly RAW, but the first one seems more thematically appropriate to me. The only reason the first approach shouldn't work from a fluff perspective is if your setting's phylacteries are minor artifacts, in which case an AMF cannot suppress them, but can still suppress the lich's rejuvenation itself.
Depending on what level you intend to have the lich be and on what level you intend to have the PCs be there are several options.
The first that pops into my mind is a permanent demiplane that has been given the dead magic quality. This requires high level magic, and you have to create the plane, make it permanent, create another part of the demiplane (which isn't permanent), and then make the whole thing dead magic and then wait in the non-permanent section to end and kick you out of the demiplane. Oh and toss the phylactery in the dead magic area.
Then you go and kill the lich. Even if the lich reforms, they're stuck in a dead magic demiplane with no portal to another.
I like to call this the prison demiplane. The only thing that can get you in and out is a wish (because it ignores local conditions, although there is argument about whether or not dead magic is covered by this clause).
Trick him into using Daywalker and then flesh-to-stone him. By RAW, I think flesh to stone's only stipulation is that the subject has flesh when you cast it. Conceivably, it should work on inanimate objects if they have flesh somehow, as the spell specifies a subject, not a creature, and doesn't specify living flesh, so undead immunity to fort saves shouldn't actually matter by my reading. Daywalker's duration running out shouldn't technically change anything, since the spell is already cast.
Meirril, I think the lich's soul is stored in the phylactery already, destroying the physical form doesn't move the soul anywhere or make it available to "catch".
This is why you typically have to get access to and destroy a lich's phylactery.
You could roleplay the prison. The players might cultivate an intellectual pursuit for the Lich--some kind of unsolvable intellectual puzzle that will occupy the Lich's undivided attention for centuries.
How are you roleplaying your Lich? What are the Lich's motivations?
Well, it's a home brewed world in which one kingdom (Kingdom A for the sake of discussion) has come into conflict with Kingdom B. Kingdom B happens to be a kingdom with powerful necromatic powers at their disposal, resulting in an army of undead.
Seeing the dire circumstances set forth by this war, my lich, who is something of a nationalist in Kingdom A, decides that the only way to combat this undead hoard is to ascend to lichdom, thereby gaining more power over undeath than the necromancers in Kingdom B.
A Feeblemind (Psychic will of the dead phrenic amplification or any caster with Threnodic Spell metamagic) or Microcosm (Psychic with will of the dead) will effectively neutralize a Lich permanently even if it is destroyed and recreates its body.
An interesting thing about Microcosm, which is a really cool spell, is that if you were to place the person in an antimagic area the effect would be suppressed, since it's duration permanent not instantaneous.