What is a dempilane? Demiplanes are a separate layer of reality that sufficiently high level mages or gods can create. While a god have more control over all aspects of their plane, even a mage has extraordinary levels of control over their planes.
Demiplanes are admittedly not the most powerful spell you can pick at such high levels, especially when set next to spells like wish, which, as the name implies, bend the laws of the universe to grant you your wish, making your creation of your own plane of reality seem relatively minor in comparison. But despite this, demiplanes are epic in my view, and this is due to the amount of roleplaying potential found within them.
Mages of generally around 17th level in D&D can create their own plane of reality, and they get unparalleled control over these planes of reality, making them a player’s paradise. For example, if you want, you can even choose how quickly or slowly time passes in your plane (within reason, you aren’t a god yet), but this gives you more time to do things like research about your upcoming opponents, or simply lounge around, you are playing a mage after all, meaning you work hard and play even harder.
Not only all of the above, but a demiplane is about as secure as it gets, seeing as you control all landing of opponents, allowing you to trap the place so thoroughly that you can kill gods, and then, in the time-honoured tradition of all role players, loot everything they own while laughing manically about how much all the new stuff you got will improve your character.
Due to this, demiplanes, in my mind, are epic. Having a secure base that I have absolute control over is something difficult to accomplish for players, something I can attest to due to the number of times I’ve been attacked or robbed in my own base.