Uncontrollable Power

How do you write/play a character with incredible powers/abilities they cannot control properly?

I wouldn't, because characters like that are pure garbage in any medium.

But if I had to, I'd probably use something similar to a miscast table where random uncontrollable bullshit happens every time you fail your "control" roll.

You make them mad as hell and give them a weird body horror arm.

I would use such a character as antagonist. In my homebrew Cape setting people like this are my Main antagonists.

Do they begin as antagonist, or just end up there?
even as an NPC?

They're probably likely to be quite scared of their power, or very cavalier about it (especially if they're a bit sociopathic)

As a "force of nature" kind of villain, who's long since devolved into acting on pure instinct or survival reflex.

Otherwise echoing what said. Those kinds of characters are garbage in any medium they're in. It's just an excuse for angst and "wahhh, I can't control myself!" where the character either learns control and becomes a god or doesn't and you're supposed to feel bad (in a really forced and ass-pulled way) about having to take them down.

>write
The usual I guess, tries to point them at the bad guys, doesn't always aim it right.
>play
High damage spells that are hard to aim, such as trading off attack bonuses to rolls for extra damage dice.
As for spells that don't aim and are area of effect dex saves or something like that, add in a scatter dice and roll for where the attack goes in comparison to where they aimed.
Also forbidden from healing magic, Cure Wounds becomes Inflict Wounds by default in their hands. They pour too much positive energy in you and you get cancer.
As for buffs, they might negatively impact other aspects.
Mage Armor gives a huge bonus to AC but you get disadvantage on stealth and lose some of your dex bonus as if it was medium armor.
Shield blinds you as the disc of force explodes into existence.
Spells that buff stats debuff other stats, and boosting strength causes physical damage.
Spells that grant useful powers give problems as appropriate.

>even as an NPC?
They can make for useful plot devices, but as characters they always end up shit.

The problem is that "uncontrollable" is always an illusion. After all, whether they're a player or a villain or a generic NPC, there's always someone at the helm. That's why I'd use a miscast table so you can at least have some actual randomness by rolling on the table, which is the only balanced/objective way of doing a character like this.

If it's a player, you're essentially allowing someone to be overpowered with nothing but their promise that the "random" downside will balance things out. Nine out of ten times, this means that the downside will only be relevant when it's convenient, either mechanically (downsides only come out when it doesn't cause any serious problems) or narratively (downsides only come out when the player thinks it would be cool/interesting). I'd expect character types like this to be common on those freeform anime roleplaying boards, because it's an extremely safe way to be "unsafe" if you catch my drift.

As an NPC you've got the same issue except on the GM's end. Which is why they make a good plot device but not a good character. You know, like the guy just happened to go nuclear while the party was in town and now you got to stop him, or something like that. As a character, the players will always be acutely aware that the lack of control is just an excuse for the GM to pull stuff out of his ass (good or bad) and whether the character is in control or not (and what happens when he isn't) is nothing more than the GM's whim. If the situation was real, there'd be a huge narrative tension centered around the character's lack of control. But since it's not real and everything comes from the mind of one guy, it just becomes another part of the plot and there's no real tension anymore.

That might be a bit too meta, but I don't see the point in a character narrative that relies on randomness/uncontrollability as a source of tension.

>If it's a player, you're essentially allowing someone to be overpowered with nothing but their promise that the "random" downside will balance things out.
Just gotta use something like GURPS' disadvantages and phobias.
Have the player roll against some number, always the same, decided at chargen. If he passes, great, his power's under control but doing that is distracting and he has penalties to DX and IQ related tasks. If he fails, he blows up. And roll on that for every encounter, including social.

Yeah, that's similar to the miscast table idea I mentioned. I just don't think that's a very fun way to play.

TETSUOOOOOO

Having done so before, it's all about discussing it with your GM and figuring out the best way to do it.

Generally I statted them as a normal character to represent them holding back, with a disadvantage of some sort linked to accidental and destructive side effects of the power, and sometimes an 'overcharge' ability of some sort to represent them trying to let loose intentionally.

Overall it's pretty fun. Making the expression of the sheer scale of their power a disadvantage/negative is key to it being fun for the rest of the group, though. It's also interesting to fluff actions in combat based on someone doing their best to hold back. I also really enjoyed roleplaying the drama and trauma in the moments after they let loose, realising what they'd done.

They start as antagonists. They are essentially people who became too much power when the powers manifested im people. There are three different categories. Class C are the strongest but they also can't control their powers and are severly mentally shattered to a point where they can't even communicate with normal people.

By keeping them out of the plot entirely. In my setting there actually is a superhero with uncontrollable powers that essentially causes reality to fuck itself while the hero himself has toon force powers. The only reason no one stops him is because as often as his powers can cause damage, he protects the entire state of Florida extremely well by himself. The issue is his powers also have made him real fuckin' weird, so no one wants to work with him.

He literally just exists for comical headlines with "Florida Man" in it.

CANADAAAAA

>He literally just exists for comical headlines with "Florida Man" in it.

No seriously, his superhero name is Florida Man. It was originally something else, but then the brain damage from his powers just made him decide he was the hero of Florida... which works out well because he has a minor precognitive power that lets him determine threats so long they are within the exact state boundaries of Florida.

Suffice to say, no hero goes to Florida. Not even the Superman equivalent.

Give them real high skill boosts and an extremely low Fate point refresh. Done.

#SOTC4LIFE

A mechanically more interesting way to do the same thing would be to have the miscast table be an escalating scale of consequences. Every time the PC wants to tap his dangerous power, he has to choose how much and roll on the table. More power drawn means more dice / higher modifiers, and therefore more risk.

Give the PC a benefit proportional to how deep he goes down the well, but it's an actual choice. At max power, he can overcome any obstacle.... but the numbers way up on top of the miscast table are all extremely bad.

This is actually kind of how Don't Rest Your Head works, as a system. Any PC has access to more dice than any problem they could face- But the more dice you roll, the more and more likely you are to suffer severe consequences because of it.

It's a fascinating dynamic to use in a horror game, because instead of the usual horror of powerlessness, it's almost the terror of knowing that you Are strong enough to do it... But it's a question of how much you're willing to lose along the way. It's kinda fascinating to run as a GM, because you don't really need to do anything. The system perfectly enables the players to, slowly and surely, destroy themselves.