Damage as rolled so roll 18 do 18 damage.
I believe you could empower a low roll to both cast and/or increase the damage, so need a 12 roll a 10 empower by 5 gives 15 and that Major Demon is toast. It's one of the spells where lowering the cast number helps to cast but reduces the effect of the spell, like Mind Control and all the Will opposed spells.
I believe scrolls always 'go off' at the base casting number so 12 damage, just enough for a Minor Demon but not for that Major Demon.