Complex sentence: When a kelpie appeared in its equine persona it could be captured using a halter stamped with the sign of a cross, and its strength could then be utilised for work such as transporting heavy mill stones.
Split into simpler sentences: When a kelpie appeared in its equine persona, it was possible to capture it using a halter stamped with the sign of a cross. Its strength could then be utilised for work such as transporting heavy mill stones.