...A bag of holding and a portable hole both use slightly different methods of sidereal access to the ethereal plane in order to function. It that it is due to destructive resonance of the incompatible and differing methods of accessing the same plane which causes the spectacular interface effects that have been noted in the journals and materials referenced in your handouts. To provide an analogy, it is somewhat like attempting to march a company of soldiers across a wooden bridge in perfect lock step.
As a counter example the effects of a rope trick, psychoportative shelter, or magnificent mansion all use non-dimensional space. As such, there is no interference potential between them and a bag of holding or portable hole.
Furthermore, there is no indication in any of our recorded observations that said bags of holding or portable holes within one of their own kind would not function as normal. In fact, the amplification potential of two similar energy signatures would theoretically make it quite difficult to remove the secondary item from within the first, once one item was placed within a similar example of its own kind. And by extension, a sufficient chain of like items within each other would amplify the energy signature to such a degree that it would quite possibly strain and potentially rupture the materials out of which the items are constructed
It is important to note that the non-dimensional space and the claudication adjacent to the normal space inside a bag of holding or portable hole is not shunted aside or somehow made non-existent when the claudication is suppressed. The space in question has been extended through said claudication with the effective addition of the non-dimensional space. Thus any object placed inside such a bag or on the surface of such a hole in a power-dead region (such as naturally occurs in certain locations, please refer to Figure 7.4 in your handouts, and as also can be artificially constructed such as the famous, or infamous, anti-magic field), such an object would simply fall through the reopened claudication into the extended space once removed from said power-dead region....
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...Non-dimensional space (as the term is confusingly used interchangeably with the similar term extra-dimensional in the source materials referenced in your handouts, though for reasons based upon my own studies, I prefer the former,) are naturally occurring areas of potential weakness in the weave of the fabric of space-time with the specific quality of elasticity. Not all such areas have this quality of elasticity. Other areas with differing qualities, and therefore utilities (such as rigidity and the commonly and popularly associated dimension door) do exist, but that will be covered in next week's lecture.
This quality of elasticity allows, with the correct application of energy, such an elastic area to be stretched and expanded into a closed pocket of space-time with limited dimensions, and characteristics of the same space from which it is formed. while such "bubble" are of limited utility in and of themselves, with the exception of certain applications in mutli-dimensional / multi-planar construction and precise positioning, if a stable claudication can be adjoined to both the "bubble" and a more normal section of space-time, an accessible pocket-space is formed. common uses include living quarters for various prime and planar entities, both sentient and otherwise, and storage locations for substances and items found useful and valuable to the same....
regarding the hiding of a phylactery, and as has been mentioned at least twice already in the thread: within a living being, so that their living signature cloaks the undead signature of the phylactery, is a fairly good choice. on the other hand, using someone with the that feat which makes them be healed by negative energy might also confuse or mask the signature. see if the lich can manage to capture a pc, and place some sort of destructive contingency on it such that removal equates unavoidable and unrecoverable death for the pc in question. maybe a chain of contingency where removal of the phylactery slays the pc, and traps the soul, and when the soul gem is filled the gem and phylactery teleports to the lich or a predetermined location.
i also like the idea of giving it to a bound powerful and dangerous being, then using imprisonment on it.
one of my personal favorites is to make a permanent item of contingency-time hop either attached to or out of the phylactery, with the stipulation that it triggers if anything moves within the detection radius. thus it will jump into the future in near perpetuity: the wind rustles leaves or grass... jump... any living or undead thing moves... jump... a shift in light or shadow... jump...
depending on how things turn out, it may last until the local star goes nova. might want to attach energy protection - all to it.
in a similar vein, shoot it into orbit, or into space, or "accidently" lose it during a spelljamming trip.
