The Book of Leinster
The Book of Leinster records several cases of Druids taking opposite sides in battle. It was Greek meeting Greek. The northern Druids plagued the southern men by drying up the wells; but Mog Ruth, of the South, drove a silver tube into the ground, and a spring burst forth. Ciothrue made a fire, and said a charm with his mountain-ash stick, when a black cloud sent down a shower of blood. Nothing daunted, the other Druid, Mog Ruth, transformed three noisy northern Druids into stones.