This is a proposed system for using classic AD&D spells in Eternal Soldier.
All spell types -- Clerical, Druidic, Illusionist and Wizard are just different types of magic for purposes of this system. It's up to the GM what extent he or she wants to tie them to a particular cosmological view. This doesn't change the way they work for game purposes, however.
Each spell type is a different skill.
Druid: 2 Int, 3 Wil
Cleric: 4 Wil, 1 Cha
Wizard: 5 Int
Illusionist: 1 Dex, 2 Int, 2 Cha
The difficulty of a spell is determined by its level. The chance of successfully casting is the caster's skill percentage -(spell level x 10). So the caster has a -50 to their chance of successfully casting a 5th level spell. Obviously, 0 levels spells can be cast with no minus.
Adjustments may apply if the caster is setting or quick acting in combat. See "Casting Time" below.
The caster may take extra minuses on the spell to cast it as if it were being cast at a higher level. Normally a spell will be cast as if it were being cast by a caster of the base level necessary to learn the spell -- i.e., fireballs, which require the caster to be 5th level in AD&D, are cast as if by a 5th level wizard. The caster may take an additional -5% for every additional level he or she wishes to cast the spell at. For instance, fireballs are normally -30% (3rd level) and go off as if cast by a 5th level mage. The cast may take an additional -10% (-40% total) when casting, and if he or she succeeds the spell will go off as if cast by a 7th level mage.
The time it takes to cast a spell is determined by the casting time in the player's handbook. If the casting time is in segments, then the number of segments is the speed class to cast the spell. If the casting time is in rounds, the number of rounds is the number of Class X actions necessary to cast the spell. If the casting time is in turns, this is the number of 10 minute periods the spell will take. Any other listed time is the time necessary to cast. Bonuses for quick acting and set acting as per Eternal Soldier initiative rules can only be applied to spells which have the casting time in segments.
The following rules apply only to wizards and illusionists. Clerics and druids can attempt any spell they are of sufficient level to try, but can only cast a total number of levels equal to their combined skill points, plust will and charisma bonuses in one day.
If a caster is of sufficient level to have a chance of casting a specific level of spell (i.e 10% for 1st level spells, 20% for second level spells, etc.) then he or she can attempt to add spells of that level to his or her spell book. Whenever a skill point is added to the casters percentage, the caster can attempt to add a desired spell from each level. The caster must make an intelligence roll to succeed in adding the spell. If the intelligence roll is missed, go to the next desired spell and try adding it. And so on. If the character blows the rolls for every unlearned spell of the level, tough.
If the character has the "write" spell, he can also add spells to his or her book by copying them from a scroll, a tome, or another mage's spell book. A successful "write" cast is necessary, in this case. If failed, the caster cannot reattempt to write from that copy of the spell.
When a caster wakes up refreshed from a decent nights sleep, the first thing he or she does is memorize the spells from the spell book that will be available that day. The number of spell levels that can be memorized is equal to the caster's skill points in the given skill plus their intelligence and will bonuses. That is to say, a caster with an 18 intelligence and an 18 and 8 skill points in wizard could memorize up to 16 levels of spells for the day -- 1 9th level spell and 1 7th level spell, or 16 first level spells, or 3 5th level spells and 1 1st level spell, etc. As spells get used for the day, they are no longer available until they are re-memorized the next day. The caster can memorize a particular spell multiple times, for multiple uses during the day.