[apoger]

>Couls someone briefly explain what communion is & how it works? What makes it so powerful?

In Brief:

Some mages cast Communion Slave while others cast Communion Master.

The Slaves take the fatigue when the Masters cast spells. This allows a great deal of casting by the Masters.

The Masters also get magic skill bonuses from the Slaves. All skills +1 for 2 Slaves, +2 for 4, +3 for 8, +4 for 16.

For example:
An Arch Theurg (air-2 astral-3, lets say the ? comes in as nature-1) goes into battle.
There are 8 Communicants with him.
The Arch Theurg casts Communion Master.
The Communicants are slaves automatically.
The Arch Theurg is now air-5 astral-6 nature-4.
When he casts spells, he does not fatigue, the slaves soak up the efforts of spellcasting, allowing the Arch Thuerg to use his pumped up ranks over and over again.

More Slaves will absorb more fatigue.
More Masters will gain the full benefits of the Slaves. The Slaves are shared.
The only danger is driving the Slaves fatigue beyond 200, at which point they take damage and die. Burning out the slaves is common in long battles, however it gets you a stunning supply of mage power before they drop.