[Cohen]

Does it really raise magic skills in battle?

While other spells raising magic levels (like Power of the Spheres and so on) are noticeable, if you keep an eye on you mages, you'll notice the paths raised.
With communion, you don't see them raised.
And slaves get a lot of fatigue, so I suppose magic paths aren't raised and so spells are casted at full fatigue, while with 4-6 slaves you should get +2 in all paths, and fatigue should be far lesser.