Paper And DiceAlright, most unique miniboss fight in my #DnD5e career (and probably my entire DnD career) during tonight's session. The party had to fight a sapient disease that had spent the past 2 weeks infecting everyone but the paladin. The mechanics of it worked this way: Any infected character was poisoned. At the start of each round of initiative, all infected characters had to make a Constitution saving throw. On a failure, the character was puppeteered by the disease for the rest of the turn.
All infected characters took 4d6 poison damage at the start of their turn and had to make another Con save at the end of their turn or gain a level of exhaustion. The disease could take 2 actions per round, taking them on the turn of a puppeteered character. It could make a single weapon attack without the benefit of the character's class features or feats, but with +2d8 radiant damage.
Or it could cast one of its clerical spells (because the sapient disease was a cleric) from its list, which included banishment, insect plague (upcast to 7th level), and contagion from what the players saw. It was still limited in concentration, but could cast multiple per turn. Or it could exhale a cloud of infectious disease onto nearby creatures, forcing them to make a save or get infected. Other actions may have been possible, but the players never saw them.
Fighting back required effects that could cure disease, which the party cleric and paladin had available. Other effects, such as things that blocked possession or something like dispel evil and good would have worked as well. Curing the disease required the person doing the cure to make a saving throw using their spellcasting modifier. They never failed this in 6 curings, so they never saw what would have happened on a failure (a bunch of psychic damage to the person getting cured).
I only decided to pull this kind of thing off because I had multiple characters with magical disease countering, but it was a lot of fun and everyone was trying to figure out the mystery of it as things went completely off the rails. This happened way earlier than I expected as well, because they were about to eat a Heroes Feast and I realized that cured disease. It wasn't intending on revealing itself, but faced with the choice of kill or be killed it chose kill and lost the battle.
Oh, also, curing the disease removed any exhaustion levels. While the party cleric got to 3 levels before she finally had a round where she wasn't getting puppeteered and they managed to cure her, that was the worst it got. Plus a few party members down a lot of hp. The paladin took a beating because the disease wanted him dead. As the only person immune to it and with the ability to cure it, he was priority number one.