Guide: Guide to communions

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Depends on the spell mostly. If you're spamming low level evocations your magic path (boosted through the communion) is high enough, you may only be giving your slaves like 4 fatigue. In that case it should be fine for the slaves to cast themselves to 100 fatigue, and then it will still be a long time before they get to 200 and start taking damage.

On the other hand if you're casting battlefield wide spells that have several hundred fatigue attached to them, you probably don't want your slaves casting additional spells.