[lch]

While the spells themselves are zero fatigue, the action of casting usually is not. See the actual value by clicking on the "Encumberance" stat for the slaves. Plus, if the slaves are casting spells for which they don't have the right magic paths and levels, for example holy spells when they themselves have no holy magic, then they get additional fatigue just like the manual lays out. I think that when they don't have holy magic paths at all, they don't benefit from the communion bonus, too, but I am not that sure on that one.