Just Happy Little Accidents

Magic is almost always a gambit. A magic-user's limit should be how many spells they can SAFELY cast before the tides of chaos bring them down. Arcanum in Into The Odd are really great, and I love the rules as a whole, but the system lacks miscasting. The closest thing that it has to miscasts is a brief blurb about how some arcanum can have side effects (such as cursed items, but these are a tad different from miscasts) and identifying arcanum (when characters pick up arcanum, they can sense its nature but not its specific power. Also they must pass a WIL save to understand its power before using the arcanum at first, if they fail then the arcanum accidentally activates) which are all good, but not the miscasts I'm looking for.

Into The Odd lists three example "types" of arcanum, "big arcanum" (REALLY big stuff, like magic couches), "disposable arcanum" (potions, bombs, you get the idea), and "arcanum weapons" (arcanum that are weapons alongside their abilities) So, I'll give two custom "types" of arcanum that support miscasts.

Creepy arcanum. This post doesn't need art, but here it is anyway! (Jack Of The Dust)

Limit Arcanum

These arcanum are based on probably the best post in the entire OSR blogosphere (it is, fight me), Small Tables. Basically these arcanum have an ability that can be used safely only once per day. Any usage beyond this requires a D6 roll on a miscast table related to the arcanum. The arcanum still works and uses its intended effect, but there is a screw-up after its ability is triggered. The results 1-3 on the D6 table are minor miscasts, all grouped up into one result. The results 4-5 are major miscasts, these are two separate results that are major detriments to the magic-user. The 6th result is a miracle, the arcanum works as intended, this time... Here are three example limit arcanum:

Cackleskull

The crystalline skull of some creature or another. If its mouth is opened, it begins to cackle madly. Everybody that the skull is pointed towards while cackling must make a WIL save or else break out into laughter that sends them to the floor for 1D6 minutes. If they succeed the WIL save, they break out into laughter for half of the time. 

This can only be used safely once per day, if it is used over this limit, roll 1D6 below.

D6	Miscast Result
1-3	The skull is way too loud, roll two random encounters if one is available. Otherwise, pretty much all stealth is ruined and this attracts lots of attention.
4	The skull begins to float, cackling madly. It randomly spins, using its ability once every minute for 10 minutes. It also shoots razor-sharp teeth at anybody who is within 5 feet of it for 1D8 damage.
5	The skull reveals a mass of wriggling purple tongues that grab the caster and 1D6 extra targets. All targets are pulled towards the skull, the skull will attempt to eat the targets.
6	The arcanum works as intended, this time...

Phoenix Mantle

A mantle embroidered in orange and yellow feathers, it kinda makes the wearer look like a bird. On a mental trigger, the user can make their body white-hot for 10 minutes. This creates a pillar of smoke around the user, makes all fire attacks against the user impaired, and anyone within 5 feet of the user takes 1D6 damage at the end of their turn from the heat alone. 

This can only be used safely once per day, if it is used over this limit, roll 1D6 below.

D6	Miscast Result
1-3	The user begins choking on the mantle's smoke and squirms under the heat, they take D6 STR damage and cannot see as they cough on the floor.
4	The mantle shoots out a pulse of flame. Everybody within 10 feet of the user immediately lights on fire, taking 1D6 damage every turn until they put it out. The mantle sounds like it's screaming, ruining all stealth.
5	The mantle randomly starts shooting fireballs at random targets for D10 damage, the user begins burning alive.
6	The arcanum works as intended, this time...

Sword Of The Shackled

A rusty shortsword, there is a weird clinking noise within it. On the hilt is a button, if this is pressed then the blade of the sword shoots out 30 feet, connected by a chain. The button can be clicked again to pull the blade of the sword back onto the hilt. this blade does D6 damage, but it does D8 damage if it pierces somebody when the blade flies through the air.

This can only be used safely once per day, if it is used over this limit, roll 1D6 below.

D6	Miscast Result
1-3	The chain refuses to retract, the blade is now either thoroughly stuck within the surface it hit or bounces off the surface harmlessly. Choose whichever one is most inconvenient for the situation.
4	The chain is very powerful, pulling the user towards the surface that the blade struck or dragging the surface forcefully towards the user. Choose whichever one is most inconvenient for the situation.
5	The chain is loose, the blade ricochets off 1D6 + 2 surfaces. This can cause the blade to strike allies, enemies, or even the user. At the end of the ricocheting, the chain breaks.
6	The arcanum works as intended, this time...

Gambit Arcanum

These arcanum are really risky because each use of it could trigger a cataclysmic failure. Basically, the arcanum has a recorded "maximum score" that is kept secret from the players. Every time the arcanum is used,1D6 is rolled, recorded, and added onto all other D6s rolled this way. So, for instance, somebody uses the arcanum once and rolls a 6, the item's recorded score is 6 at the moment. The second time the arcanum is used, the user rolls a 3, the item's score is 9 now, and so on. Once the "maximum score" is met or passed, the arcanum triggers a Cataclysm that is unique to it. These cataclysms are often, well, cataclysmic scale miscasts. They are massive detriments to the user and possibly the people around the user. This of course requires more bookkeeping but adds lots of suspense in using these arcanum along with a neat and weird effect triggered with gross misuse. The arcanum can often not be used after their cataclysm is triggered. Here are three example gambit arcanum:

Tearcap Mushroom

A bulbous black mushroom that kind of looks like a mix of a fat toad, a melting candle, and a ball of mold. Every time the mushroom is squeezed, it spews forth prismatic spores that go into the noses of everybody within 30 feet of the user. Everybody within this radius must make a STR save or else begin hallucinating for 10 minutes. This 10 minutes begins counting down once the victims get out of the spore cloud. The user can reroll their STR save if it fails once.

Every time the user activates this arcanum, they roll 1D6 and record this result, adding it onto all other D6's rolled this way. Once this result is equal to or higher than 14, the spores fill the entire room the user is in and the user permanently hallucinates as they gleefully cram the mushroom into their mouth. The user grows 1D6 mushrooms on random parts of their body per week, these mushrooms are not magical, but release hallucinogenic spores a maximum of once per day.

Doppleganger's Face

A face made of sagging purple skin; it has 3 eyes, a mouth on its forehead, and 4 ears. While on somebody's face, the wearer can mentally command it to change into any face that they have seen. The user must pass a WIL save or else there is one minor detail wrong/off in the face.
Every time the user activates this arcanum, they roll 1D6 and record this result, adding it onto all other D6's rolled this way. Once this result is equal to or higher than 14, the face grows spindly spikes and digs itself into the face of the user. Every day, the user rolls 3D6, this is their new WIL score, the GM decides what happened to the user's face. If the WIL score equals 7 or lower, the user is gone and is now possessed by a doppleganger.

Broombug
A giant bug (12 STR, 14 DEX, 4 WIL, 6 HP, D6 Proboscis, Can Fly) that looks like a broomstick. The broombug will follow any commands for 10 minutes once per hour. It can hold up to two people, but no more. It can also fly up to 30 feet in the air. Once the 10 minutes of commanding it are up, the broombug becomes lethargic and shuts down.
Every time the user activates this arcanum, they roll 1D6 and record this result, adding it onto all other D6's rolled this way. Once this result is equal to or higher than 12, the broombug stops mid-flight and begins to spasm. Two pairs of wet wings reveal themselves as it shivers and grows two new heads. It now goes into a blood-cra frenzy (14 STR, 18 DEX, 4 WIL, 12 HP, 1 Armor, D6 Proboscis, Can Fly)

Nothing like cataclysmic growth of mushrooms in the morning.
(Eric Guidry)

Telegraphing Danger
The main concern with this kind of miscast system is how the players will know that the arcanum is dangerous and can be miscasted. With the use of proper telegraphing, this can be achieved. Here's a few responses to give players to propely tell that the magic item they're using is getting more dangerous.

After a limit arcanum cannot be used safely for the rest of the day:

• "You can see that magic is ebbing from the arcanum, any attempt to use it further for now will probably be in desperation."
• "The arcanum silently shivers in your hands, it'd surely be reckless to use this again anytime soon."
• "As the arcanum is used, you feel its magic fleeting for now. Whatever resivoir of magic it has left is chancy at best."

After a gambit arcanum is getting close to either reaching or surpassing its "maximum score":

• "The arcanum excitedly quakes in your hands, something is inside of it, waiting to be released."
• "A pulse can be felt from your arcanum, it wants to be used again, it wants release."
• "The magic from your arcanum releases in a violent burst, something dangerous is bound to happen if it keeps on doing that."

Generally for limit arcanum you can describe the magic getting more unstable after its safety net is used up, or you could describe that any attempts to reach into the arcanum's power is likely desparate. For gambit arcanum you can describe the magic of the arcanum itself getting more wild, with the item tearing apart at the seams as it reaches its inevitable cataclysm.