The biggest eyebrow raiser was the INT bonus. It surprised me to learn that the INT bonus provides a number of spells equal to the bonus at each spell level, as soon as the ability to cast that level is obtained. So, a mage with a +2 INT bonus, upon obtaining 5th level can suddenly cast three spells. That combined with the free-form nature of casting seems to make mages more powerful than their traditional OSR ancestors. That's not necessarily a criticism, I'm just wondering if it affects play.
My favorite thing about ACKS magic, bar none, is the flavor. According to the flavor text:
For an arcane spellcaster to have a spell in his repertoire, he
must keep track of complex astrological movements and star
signs that are constantly changing; he must daily appease
various ghosts and spirits that power certain dweomers; he must
remember and obey special taboos that each spell dictates. All
of these strictures, and they are many, can vary with the season,
the lunar cycle, the caster’s location, and more. Having a spell
in the repertoire is thus an ongoing effort . . .
So, it isn't a matter of constant re-memorization for an ACKS mage. When he has his nose buried in his spellbooks he is actually checking to see which planet is over his left shoulder and the affect it will have on his Sleep spell. Very cool to me.
Oh yes, that brings me to the repertoire. This is the concept whereby free-form casting works. Mages in ACKS still have a chart showing number of spells per day. Now that I look at the chart closer, it is a bit slower progression than some others, notably the LBB, but the same as Dark Dungeons and Labyrinth Lord. Maybe that INT bonus I mentioned earlier is intended to provide a little something extra to mages with the right stuff.
Anyway, the familiar spells-per-day is now the mage's repertoire. It represents not only how many times per day he can cast spells of a given level, it also represents how many he may "keep in mind" of a given level. For example, a 1st level mage with a +2 INT bonus has three 1st level spells in his repertoire. He may cast three first level spells per day. Let's just assume he has Charm Person, Light, Magic Missile, and Sleep in his spellbook. He would designate three of the four to be in his repertoire, meaning he could cast any of the three, as desired, up to three times per day, total. So, the player still has to think about which spells he believes will be most useful, as with the Vancian system, but he has a little more wiggle room when doing so. No more using all your 3rd level slots on Fireballs only to discover you really Water Breathing.
As an aside, if you happen to like ACKS, but prefer traditional Vancian magic, it would be easy to use it, since the spells-per-day chart is already there.
The last thing about the spell system I'm going to cover in this post is spell signatures. They represent tell-tale signs in how a spell actually manifests. Is your mage's Magic Missile shards of glass, or maybe tiny laughing skulls? Whatever it happens to be, it has no mechanical effect on the function of the spell. It is possible to divine something about an unknown caster from studying the signature. Signatures can vary by campaign, meaning that they can be based on the individual, magical philosophy, the college where your mage studied, or any other factor you can think of. So, in my world of Aranor, signatures would be based on college. I suppose within that "college signature" framework it is acceptable to allow individualization, such as the shards-of-glass magic missile being a certain color for a certain mage.
Anyway, there you have it, my first impression on my first read of the magic chapter. More to come as I continue my way through ACKS.