I did it slightly differently in my campaign. I have a feat that allows characters to learn spells, but after the first one they have to spend Skill Points to learn them. And both their casting level and the levels of the spell depend on how many ranks of the skill they have.
For example, there's a bunch of larcenous slackers in my game called the Darklings, who get limited access to some darkness-related spells. I had to invent a lot of spells to give them some choice, so many of the spells listed are custom (and even so I ran out at the end). But the important thing is the relationship of ranks to spell level, which I think is well balanced.
IMC spellcasters have to make "Spellcasting Rolls" to cast their spells (don't worry, I give them some other advantages, and the players feel it all works out), which should explain why the spells here have Skill Checks associated with them. But that can be changed for different campaigns. Obviously, this technique can be adapted to other types of spells: plant spells, or illusion spells, or air spells, or whatever matches the character concept. But you might have to make up some spells to have at least one for each rank.
NEW FEAT: DARKLING SPELLCASTER
¶ You are a member of the informal group known as the Darklings, and have some small ability to cast darkness-related spells.
¶ Prerequisites: Wis 13, non-lawful alignment, Bluff 2 ranks or Hide 2 ranks, training from Lamont, "Duke of Twilight"
¶ Benefit: Darkspell is a cross-class skill for you. You receive 2 free ranks in that skill.
¶ Normal: Without this feat or a similar ability, you cannot gain ranks in the Darkspell skill at all.
¶ Special: "Training" can be quite informal, and may, in some cases, amount to no more than having a few conversations with Lamont. He must, however, agree to train you in order for you to get this feat.
NEW SKILL: DARKSPELL (Cha; Trained Only; Darklings Only)
¶ You can cast a small number of darkness-related spells. Each time you gain a new rank in this skill, choose one spell from Table I: Spells. Your choice is restricted based on the rank you're gaining. In addition, you must have a Charisma of 10 + the spell level to choose a spell. You can always choose a spell of lesser level than your rank or Charisma score would otherwise permit.
Once you choose a spell, you can attempt to cast it once per day. If you choose the same spell more than once, the number of times per day you can attempt to cast it is equal to the number of times you chose it.
¶ Check: In order to cast a spell, you must make a skill check vs. DC 7 + 2 X the spell level. If you fail, the attempt is wasted without affect. This check replaces the normal spellcasting check.
Your casting level is equal to your number of ranks. The spell's save DC (if applicable) is equal to 10 + the spell level + your Charisma modifier. The spell is cast as an arcane spell; it may provoke an attack of opportunity, and may require verbal, somatic, and material components, as usual. If you are wearing medium or heavy armor, or carrying a shield, the usual chance of arcane spell failure applies.
¶ Special: This skill is a cross-class skill for characters with the Darkling Spellcaster feat (or other abilities that grant this benefit). Such a character is called a Darkling. Other characters cannot gain ranks in this skill at all.
A character with the Darkling Spellcaster feat receives 2 free ranks of Darkspell.
If a Darkling becomes lawful, he may no longer gain ranks in this skill, and he suffers a -6 penalty on all Darkspell checks.
¶ Table 1: Spells
Darkspell Rank/Spell Level/Spells
Rank 1/Level 0/Spells: Black frogs, suppress light, ink ray
Ranks 2, 3/Level 1/Spells: Darkness missile, ebony weapon, lesser darkness
Ranks 4, 5/Level 2/Spells: Darkness missile, ebony weapon, lesser darkness
Ranks 6, 7/Level 3/Spells: Deeper darkness, wall of darkness
Ranks 8, 9/Level 4/Spells: Ebony instrument, blind contagion
Ranks 10, 11/Level 5/Spells: ??? [still working on it]
