Upper_Krust
Legend
Hey paradox mate! 
One possibility is that they could be battling a Trithemian (Thrice-Cursed) which is an Entity of Fate and Thought, Its like a weird 3-sectioned monster (in fact its sort of like a composite monster made of three parts...think Emerald Weapon if you know what I mean by that). Its USP is the threefold effect, when it hits you - you feel the effects three times, when it uses a special attack you must save three times etc...of course you also need to kill it three times, it only gets hurt on the third attack. You get the idea.
I got the idea for it from an old Thor annual which had this totally bonkers monster illustration - email me and I'll scan a picture of it (tomorrow) and send it to you. It's so weird I can't even describe it to you.
That'll give you a rough idea of what a Trithemian will ultimately look like, although i am sure I will chop and change things a bit.

paradox42 said:U_K, assuming you read through the above, do you have any recommendations for what sort of Sidereal-level monster to use for the plane's mind? Obviously one key snag is that the entity may have Cosmic String and thus be impossible for the PCs to destroy, unless they acquire an artifact that is capable of bypassing that ability (which may provide the reason they need to adventure in the post-apocalyptic wasteland for a while now that I think about it).
In any case, that's my current long-term plan for a slam-bang finish to a truly Epic campaign. There are several key sticking points, particularly in what happens if the PCs TPK or manage to pull out stops I hadn't anticipated and destroy the plane prematurely, but a DM always has to be able to roll with the punches at times like that, so I'll survive. Hope you readers found it interesting and/or inspirational.
One possibility is that they could be battling a Trithemian (Thrice-Cursed) which is an Entity of Fate and Thought, Its like a weird 3-sectioned monster (in fact its sort of like a composite monster made of three parts...think Emerald Weapon if you know what I mean by that). Its USP is the threefold effect, when it hits you - you feel the effects three times, when it uses a special attack you must save three times etc...of course you also need to kill it three times, it only gets hurt on the third attack. You get the idea.
I got the idea for it from an old Thor annual which had this totally bonkers monster illustration - email me and I'll scan a picture of it (tomorrow) and send it to you. It's so weird I can't even describe it to you.

That'll give you a rough idea of what a Trithemian will ultimately look like, although i am sure I will chop and change things a bit.