[thejeff]

More communicants. 8 is really the minimum safe number and even that can be risky, especially with blood. And don't bring more blood slaves than you're scripted to use.

I'd be surprised if only scripting "Communion Slave" and nothing else worked. I'd assumed they'd default to Cast Spells after the script ran out. You could try Hold or Stay behind troops. Easier to just use a Master who comes first in the turn order. Once he's cast, no slaves will.