The Horde Moon mod adds a new moon to Enhanced Celestials, increasing spawns of zombies.

During the rare Horde Moon event, the weight of zombie spawns will be greatly increased, and during a Super Horde Moon, all mob spawns will increase by a large amount as well, leading to a world overrun by the walking dead.

All credit for Enhanced Celestials goes to the original author. CorgiTaco

Just for fun, try it with The Hordes