Midnight Mist1 Sorcery
Transform all Humans, Vampires, Illusions, and Nightmares. Each creature must block creatures of these types this turn if able. Prevent all combat damage that would be dealt to players this turn. Stay inside during the new moon around here, whatever disappears in the darkness isn't likely to ever be seen again.
Gisa the Reveler2B Legendary Creature — Human Rogue
Whenever a creature enters the battlefield from the graveyard under your control, you may add B to your mana pool. If that creature has any counters on it, you may remove a counter of any type from it or place another counter of any same type onto that creature. "You're all invited to the end of the world—and the beginning of a brave new one."
Here are some designs that were inspired from mtg_alterficer on Instagram. I at first mistook them for customs, until he pointed out they were already printed designs. So, I decided to turn my suggestions into actual customs based on the alter art provided for each.
I had said that the original concept was very miss on all kinds of potential of flavor by skipping out on Vampire and also for foreshadowing into future development with the addition of an enigmatic creature type like Nightmares. As that goes, I realize it seems odd that a Nightmare would transform. Although it is spooky for a nightmare to transform into an even greater nightmare. I can almost seen a concept like this seeing Illusions transforming into Nightmares. I'd question how tacky that would look to add another creature type to the design. I haven't really decided against it though.
Gisa's design is a reprise of the previous concepts I've posted. I had considered giving all creatures that enter the battlefield from the graveyard decayed, but then this seemed much more like what I was looking for to capture the spirit of a reveler: a person who is enjoying themselves in a lively or noisy way. So she provides unlimited persist and undying and also a pseudo "necromancy spells you cast cost 1 less". The 2/1 with hexproof I think much better suits the persona of a character of her style. She's not very powerful, all her melee power comes from her sickle, but she's very technical with her magical prowess; utilizing wards, circles, and binds.
Midnight Mist, the concept of the monsters are out hunting is great flavor with an actionable rules text. However, there are a few problems. First, this shouldn't be hybrid. Black has no business being part of this card. Second, while nightmares transforming would be neat there are already horrors that do transform and would fit better. The fact that you may not want to transform some cards is a downside but one that is easy to live with.
Gisa, you put hexproof on a lot of your cards. Stop that. Not every card needs hexproof. Dying to removal is OK. Moving on, this is doing too much. Adding the mana would already be a ability with the plethora of 1 drop cards that self reanimate. The counter manipulation would be great on its own as well. Just stick with the counters part for the easy combo with undying/persist. If you insist on keeping hexproof then the easy combo is actually broken and you'll need to retool it.
lol at "it's broken to put hexproof on a 2/1 with an ability that's purely symbiotic".
Making part of a combo more difficult to interact with is always a concern. When that combo is already viable at the highest tiers of play and you make a part strictly better its a good chance its broken. You have at length explained that you don't play current magic so don't flaunt your ignorance when others tell you something is obviously a problem. I'm sure on your table a 2/1 with hexproof that makes your undying creature comeback with an additional +1/+1 counter it would never be a problem. But in the real world this would cause problems.
Look, everyone knows you don't know competitive magic by your own admission. So here is some solid advice that isn't limited to magic. When you hear something unbelievable about something you know nothing about don't think "that's ridiculous" instead think "so that's what things are like there." For more important things than subsections of hobbies you have no intention of delving into a cursory look into the subject should prove the validity of the claim.
Your argument majorly fails where it doesn't address the fact of all other pieces in the 'combo' here are potentially open to removal.
But they're not, though.
Using only cards within my physical reach, this combos with Aerie Ouphes to repeatedly kill all flyers on the board forever, even with spot removal. (EDIT: I nearly forgot, it makes infinite mana at the same time.)
I'm sure it's equally broken with any number of Undying / Persist creatures and/or sac outlets.
I agree that she is already over the top and giving her hexproof is ridiculous. A card that gives infinite mana *and* infinite ETBs/LTBs with Undying and Persist (and for only 3 mana) is just dumb. Not as dumb as the arguments given for leaving hexproof on there, but pretty dumb.
Worth noting, it doesn't work with Gravecrawler though. Since Gravecrawler is cast, it never enters from the graveyard; it enters from the stack. Taking away that one card obviously means nothing since it combos with about a billion other things. Even without hexproof, it just looks broken.