Hehe, The title had me immediately thinking of The Wheel of Time

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There needs to be risk though to such a potentially awesome spell. The bigger the circle the more powerful and risker, also harder to cast and more mana required.
Perhaps risk could be that every member of the circle tries to pass their spell casting to allow the lead/focal mage a better chance in cast the spell.
Something like :
- The focal is the first one to start the circle.
- The focal links to another who accepts the link.
- This continues with links forming until the last link is back with the focal.
- The first link after the focal casts the spell then the next and so forth with the last caster being the focal.
- If someone fails they suffer a manaburn or whatever and are damaged (most likely fatal).
- Death in the ring is bad and the dead mage is removed the two it was linked two are now linked and suffer some transferance damage (this allows for a casscading effect and heightens the risk).
- The final chance of the spell is left to the focal/leader as it checks to succeed modified by the rest of the mages success.
- Failure now loses the spell and results in a manaburn (larger?) with transference damage to the first and last link. After the focal is removed, either the circle is broken or the first link takes over? not sure...
- The best way I can see it is the chance for the focal to succeed is only modified positively, make a spell impossible for one to cast but every successful link reduces the difficulty by an ammount until it is possible.
I hope that was understandable. I don\'t know anything about the magic implementation, this may be a laughable approach. It may sound complex its would most likely be simple in play requiring practice and paitence, I guess a nightmare to implement though.
One thing to note, if your worried about the power of the ring and your an archer.... just kill the focal to stop the spell or hit one of the weaker scrawny looking mages you may kill them all.