communion (?)

[parcelt]

Newbie question: I have seen mentioned in some posts how a communion of mages can perform impressive feats on the battlefield. Unfortunately, I have no idea exactly what this is, how it works, what it does, pros and cons, when it should best be used, etc.

Would anybody know if there is a something of a guide available somewhere that explains the basics that could help me get started? (I did a brief search but couldn't find anything)

thanks in advance