Plane made to imprison a Spellcasting Dracolich.

[Nibbens]

What it says on the package.

If you had to create a demiplane prison so a high level Spellcasting Dracolich was trapped there, and wouldn't be able to get out, how would you do it?

Assume the dracolich is a Great Wyrm black dragon before becoming a lich, and can cast spells as a 20th level wizard on top of her natural draconic sorcerer spells because she's the BBEG's right hand woman.

Cheap-shots like the Imprisonment spell and temporal stasis, etc, are a no go because plot.

Ideas?

[ExLibrisMortis]

Well, dead magic, or maybe impeded/limited arcane magic and your choice of enhanced divine magic or enhanced psionics, focused on the particular bindings used on the lich. Slowed time, so that the dracolich spends a hundred years escaping, while the off-plane guards have a thousand years to counter it. And perhaps the Static morphic trait, making it difficult to change anything, which is especially good if the lich has a very low strength score (due to being in a tiny lizard body or something), because simple objects take a DC 16 strength check to move.

[Deophaun]

Well, dead magic, or maybe impeded/limited arcane magic and your choice of enhanced divine magic or enhanced psionics, focused on the particular bindings used on the lich. Slowed time, so that the dracolich spends a hundred years escaping, while the off-plane guards have a thousand years to counter it. And perhaps the Static morphic trait, making it difficult to change anything, which is especially good if the lich has a very low strength score (due to being in a tiny lizard body or something), because simple objects take a DC 16 strength check to move.
Add in strongly Good-aligned, and maybe play with the gravity for fun. Nothing says "inescapable prison" like the event horizon of a black hole.

[Quertus]

You don't want it to be able to kill itself, so... grappled, inside an anti-magic field. Or, having DR and acid immunity, swallowed hole, inside the belly of something inside the belly of something inside the belly of something inside the belly of something... inside an anti-magic field.