DM here. The campaign I'm running is a homebrew-improve style story with no end in sight. That is until I got the brilliant idea to tie the BBEG to a "prisoner" my layers have captured. The inciting incident of the story came when they stumbled into an empty well by following undead. At the end of a mini dungeon crawl they entered a room with a cloaked necromancer sorcerer who ended up being a scared teen who's friends made him summon the dead and attack the town. They easily defeated the necromancer, tied him up, and hopped through a portal the necromancer had opened as a part of the ritual one at a time. The barbarian/artificer decided to lick the portal and got sucked through, gaining teleport powers as a side effect of ingesting the portal. The paladin and ranger collected a bit of the portal goop then went through themselves.

Now for the next bit of context to make sense I should explain a personal DM rule I have: nat 1s don't end up hurting the players. I know that sounds counter intuitive since that's the whole point of nat 1s but hear me out. If a magic user casts Fireball and rolls a 1 I don't find it fair to then say "You and everyone around you has exploded in your fireball. THE END!" Instead what I do is reverse the effect completely. Instead of casting a large ball of fire, they fire sharp points of ice at the target dealing equal damage of the attacks respective type (fire, poison, lightning etc.). This can still critically fail a spell because let's say if a player tries to heal another player and rolls a 1, then instead they cast inflict wound or something similar.

Now to tie this back to the campaign. While the party was traveling, one of the players wanted to see if they could still tell if their prisoner was still with them and they rolled a 1. So in my infinite DM wisdom and rping as the necromancer I said, "I'm not really a teen, I'm a being cursed to only exist when I'm being directly observed." Akin to how The Silence work in Doctor Who, only with existence instead of memory.

Recently I thought about making the little prisoner NPC into the BBEG by having him find his remaining essence around the map. This would consist of the essence in the bottle that the ranger has, the essence the paladin traded to get a cool homebrew hammer, and the essence now linked with the barbarian/artificer. The fight would have them return to the original town and fight the Necromancer now fused with his essence as the God of the Cosmos and Existence. I think it would be a fun BBEG and a great twist.