[K]

Blood magic only works well when you have a lot of nlood slaves and a lot of Blood magic. The slave cost means that all effects cost a lot more fatigue than is strictly reasonable. Overcasting (using more gems, or slaves in this case) is very common.

The Secret to any Sabbath is after you have it set up, have the Sabbath Master cast Reinvigoration. That removes the fatigue from all S. Slaves. Then cast whatever you were planning to cast.

As for a way to keep Blood mages from casting Summon Imps....just don't give blood slaves to blood mages (or have them near on the battlefield). Give them to a scout that follows your army. Battlefield blood magic is the rarest of all magic, so don't bet the farm by investing heavily in it.

Sabbaths are just a poor man's Communion for Blood mages. Use them to cast big spells from other paths, and keep any blood magic infrequent.