2d6 for scar effect
2. At dawn each day, the spell returns to your mind to haunt you. Save versus its effects each morning.
3-4. You develop a magic allergy; whenever someone casts a spell around you, -1 to a random ability score until the end of the day.
5-6. Disadvantage on spell saves, as the ghost of the spell tries to let its brethren in.
7. Random relevant mutation (I recommend this table). You detect as magical.
8-9. Whenever you touch something, you can tell whether or not it's magical, and if the magic is related to the spell that scarred you, you know what it does.
10-11. You can sense the direction to the nearest creature that knows how to cast the spell.
12. The spell etches itself into your mind. You can (and must) cast it once per day, as a scroll.
d12 for scar appearance
1. Lesions across the parts of your body affected by the spell. They shift and wriggle.
2. You reek of the spell; the scent fills the entire room.
3. Your shadow takes on the shape of the spell and moves of its own accord.
4. Your eyes drip with glowing tears of the same color as the spell's energy.
5. A tattoo of the events of the spell's casting across 25% of your skin.
6. You uncontrollably whisper the arcane syllables of the spell under your breath.
7. The air around you shimmers like a heat haze, tinted the color of the spell.
8. Fragments of the spell protrude from your skin like horns and spikes.
9. Wearing clothing and holding items that are not the color of the spell cause you to break out in hives and sores.
10. Patterns reminiscent of the spell slowly spread on everything you touch.
11. If you stay still somewhere long enough, you leave behind a translucent impression of yourself, the color and pattern of the spell. It lasts for d4 hours.
12. All hair on your head grows long and wild, turns the color of the spell, and moves as if animated by the spell's remnants.
Famous spellscarred adventurers:
Zanna of the Violet Waste: A fighter with a famed magic scimitar. Their eyes drip stars. They hunt bounties on seers, fearing that one will see what they have seen.
Kug: An ogre who wears a belt that projects an antimagic field to help alleviate his allergy to magic. His armor and sledge are messily painted bright orange.
Reti "please don't ask about my tattoo" Korasdaughter: A ranger who shrouds her face in a heavy cloak that covers the embarrassing circumstances of how she gained the ability to teleport.
Toad-Man Evan: During a botched attempt to steal a witch's scrying orb, he was polymorphed into a toad. He got away and found his way back to human form, but still wakes up as a toad some mornings. The smell of mud and bog, however, never goes away.
Orgon of the Order of the Lone Mountain: Forsook civilization and joined an ascetic order of hermit monks after he was scarred by a Fireball. The order's techniques let him weather the fire, but each morning he awakens in a circle of smoldering ash.
Tevias, Ringseller: Got out of the adventuring business and into appraisal after they realized they could tell if items were magical at a touch. They've gotten a reputation as an honest businessperson and a good fence, even if they won't stop muttering "Power Word: Stun".