I'm roleplaying for more than half my life now, mainly as a player, DMing only a few one shoots. My regular group and I are playing a D&D 5E campaign that will soon come to an end, and our DM is planning a new campaign after that.
In between the regular DM's campaigns, I plan to run a short one. I really wanted play in a certain part of the DM's homebrew world. He said it's all right and I can actually define parts of the lore for that region.
While planning, I had the idea for an NPC, that "rules" its own demiplane - he is omnipotent and omniscient, but trapped inside it. Other people can enter and leave the demiplane without even knowing (it may be the interior of a building).
The plan was that this NPC was malicious, and he likes drama and showing off. He has built a cult around himself. He gets "killed" by the players as part of destroying the cult. He allows it, because he got bored with his cultists and is intrigued by the PCs - he wants to show off his powers. So he rises from the dead and talks to them. He would be some sort of information pool, since he knows everything everyone else in his demiplane has ever known. I imagine him something like an (chaotic) evil Djinn.
My fear is that such a character would not be convincing enough without scaring the players away, since he would be unpredictable.
How should I handle an omnipotent NPC? How can I get the players to interact with him without being afraid of just getting erased from existence?
Gods are mostly bound to their aspect or behave in an expected way (good, bad, chaotic, deviously, militaristic, etc). This NPC wouldn't have such a codex of behavior and could act in random ways.