Using Spell Dice

*NOTE: This mechanic originally came from Brendan over on Necropraxis. He deserves the full credit for its invention and proliferation. Below is simply my adjusted rules for it in my own setting. Be sure to visit his site and check out his posts. Good stuff over there. 

"Wizarding isn't a science. It's not a religion, either, despite what those soft-headed clerics would lead you to believe. It's not really an art, either. Not really. Maybe sorcery is, but not wizarding, no. Wizarding is a gamble. It's a hustle. A dodge. You're effectively gaming the system that is the universe, and like any game, you can cheat"      
               -Headmistress Rot-Tooth, Gran Remiel Witch's Academia.

"Let us consult the bones..."
               -Remlian adventurer.

Roll dem bones.

Using Spell Dice

• You can learn spells from any d20-based fantasy game.
• Wizards start play with 2d6 + wizard level spell dice in their Spell Dice Pool.
• Any number of spell dice may be used to cast a spell. Casting a spell successfully requires rolling 6 + the spell's level or higher. Any die that comes up 1 is removed from the pool. All spell dice are recovered following a long rest.
• If you roll snake eyes (three or more 1’s) your spell is a catastrophe and backfires on you, dealing damage or ill-effects to you and your party (see table below). Even if the total of the dice rolled equals or beats the spell's success number, it still backfires.
• If you roll boxcars (two or more 6’s) your spell is overpowered, doubling the damage and/or effect.
• If you roll a smash (all 6's) your spell tears open the fabric of reality to bestow a helpful boon on you (see table below).
• A wizard can also resort to using their life energy to power spells. You can expend your hit dice and add them to your Spell Dice total for the day, provided you have hit dice to spend. Once a hit dice is rolled to cast a spell, it is considered spent and cannot be rolled again that day, even if the spell is a success. 1's rolled with Hit Dice do not count toward rolling snake eyes. You can add your hit dice to your Spell Dice total as a swift action.

Example

Little Witch Akko is a third level wizard. While fighting a bugbear (like you do) she tries to cast Melf's Acid Arrow, a second level spell. Akko's spell dice total is 2d6 + 3d6 from her level, for a total of 5d6. The success roll for Acid Arrow is 8 (6 + the spell level of 2).

Akko is pretty sure she can take the bugbear, so she rolls 2 of her 5 spell dice. She gets a 5 and a 4. Success! She casts the acid arrow, rolling damage as she normally would. She gets both spell dice back in her dice pool.

Now, Akko wants to finish the job with Flaming Sphere, another second level spell. This time she rolls all 5 of her spell dice, wanting to ensure a success. She rolls three 1's, a 4, and a 2. Snake eyes. Akko immediately rolls on the spell catastrophe table and gets a 2. The spell backfires on her and she is burnt to a crisp.

Spell Catastrophe Table (d12)

1. System Shock. Caster takes psychic and lightning damage equal to the spell's damage. If no damage is listed, you take 2d6 x the spells level.
2. Reversal. Caster is hit with the spell (no save). If you originally targeted yourself, nearest enemy target is hit with the spell (no save).
3. Shrink. Caster is immediately reduced in size to tiny (6 +1d6 inches tall). Caster's armour and possessions shrink with them. 
4. Grow. Caster is immediately made 1d6 times bigger in size and weight. This growth is painful, dealing 1d6 damage x the amount bigger the the caster is made.
5. Slimed. Caster is transformed into a slime creature, retaining her/his original memories and personality. 
6. Reversal. Caster is hit with the spell (no save). If you originally targeted yourself, nearest enemy target is hit with the spell (no save).
7. Alternate Timeline. Caster's abilities are reversed. Caster must roll the opposite of what she/he normally would in order to succeed in any task.
8. Balloon Curse. Caster's body inflates to 6 feet in diameter. Caster speed reduced to 10 feet and is at -5 to attack. Caster takes 1 point of damage for every point of AC over 10.
9. Slug Curse. Caster begins to vomit slugs. Must make a DC 12 + spell level Con check to attempt to do any action other than moving. 
10. Reversal. Caster is hit with the spell (no save). If you originally targeted yourself, nearest enemy target is hit with the spell (no save).
11. Polymorph. Caster is polymorphed into a small creature. Roll a d6. 1. Kitten 2. Puppy 3. Pig 4. Goat 5. Mouse 6. Toad.
12. Soul Slip. Caster's soul is now the property of a demon/devil of the GM's choosing. Said demon/devil may call upon the caster at any time.

Spell Smash Table (d4)

1. The caster is immediately restored to full hit points. If the caster was already at full hit points, she/he is granted damage resistance for the remainder of the day.
2. The caster immediately learns the location of a nearby powerful magic item, as well as its function and use. 
3. The caster summons a helpful demon minion (monster description up to the GM, HD 5, stats as fighter) who protects and fights for the caster for 1d4 days.
4. The caster sees a vision brought on by an interested deity. They learn one important and useful campaign secret.