WorldCat: To fulfill an ancient rite of vengeance, two druids descend on New York City. Out for kicks on a dull summer night, a few Puerto Rican boys wander Central Park. Drunk, high, and bored they hack at an old oak tree, and they don't notice the white-haired couple appear behind them. Murmuring in an ancient tongue, the couple attacks the boys to save the oak. By the time the police arrive, two boys have been slashed to death, and their right hands cut from their bodies.
Source · ISFDB CC BY 4.0