It's worthwhile to read through the AST quest scripts - or at least Fortune Favours the Bole, where Leveva give you your hasty first introduction to astromancy. There are things scattered across other quests, but that covers the basics.

At its simplest, Benefic/Malefic are not related to the cards or fortune-telling, but are similar to the other spellcasters - shaping aether into magicks, as you say.

LEVEVA
Now listen carefully, for we haven't much time. The star globe which you hold is both your sword and shield. It will grant you strength, as well as protection. But most important of all, it will grant you insight into the current location of the stars, enabling you to channel the heavens' aether into your body for use in all manner of magicks.
The better attuned you become with the heavens and their “malefic” and “benefic” bodies, the better that aether might be manipulated to bring succor...or pain.