There exists an artifact known as the
Nexus, a mysterious shard of... of something. No one knows its purpose. No knows its goal. No one except
Gemini and
Alexander, and neither of them ever explained it to anyone that had dealt with them, encountered them, watched the two fight over its possession. And yet the Nexus seems to have a goal all the same. Aside from the
Starstriders who've come to possess pieces of the Nexus themselves, the original still seems to be at work.
The Portal
The Nexus has the means to open portals, gateways not unlike a
Corridor, but rather than connecting two disparate realms, they connect one world to a demiplane. The portals almost seem to know when and where to appear to attract the attention of who it wants to enter (your party), then seals shut behind the party once everyone is inside.
The portal does not open again until the party has been defeated, or the demiplane's occupant has been defeated. The term 'defeated' is used loosely - it simply means combat is over. Generally, this means one side or the other has been killed but other circumstances might constitute a win condition that permits escape from the demiplane.
The Demiplane
The Demiplane is exactly what it sounds like - the spell, with some added benefits. The interior of the space is always cube shaped, but it can be anywhere between 30 ft. by 30 ft. up to 120 ft. x 120 ft. Similarly, rather than bleak stone or wood, the interior of the demiplane may appear as if a natural environment wholly lifted out of whatever realm the demiplane's occupant(s) were captured from. Whether the room was like this before they were put in it, or the room became like this after they were put in it, no one knows - not even the occupants, who have been imprisoned inside each demiplane so long they themselves have forgotten the specifics (assuming they're even intelligent enough to make such distinctions).
Feel free to include traps, environment hazards, and other supporting effects to the encounter to spice up the conflict. They generally work more in favor of the demiplane's occupants than the trespassing party members.
As for combatants within the demiplane, there is no particular rhyme nor reason to what's been crammed in a room. Sometimes monsters that would normally kill each other on site in their native environments have been forced into the demiplane together and now work with one another to hunt whatever enters the demiplane. Sometimes unusual creatures are contained inside, like animated spells.
Special Rewards
The Nexus watches all exchanges and interactions within its demiplanes, to an end no one's quite sure of. It's not unheard of for the Nexus to deposit a lone spellshard where the portal once stood after the party exits, providing a detailed analysis of how the party approached the encounter, alternative solutions they may not have considered or vulnerabilities they failed to exploit, and other pertinent information about the encounter and the party's general performance.
On occasion, it's even been known to take a more vested interest in certain parties or party members, and may seek them out for more... critical analyses.
(And yes, somehow the party still gets their normal post-encounter rewards. Be creative.)
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