Similar to the Wandering Trader, the Purifiers spawn randomly anywhere. They are a group of 3-5 Villager-like mobs in Gold Armour, riding Horses that are also clad in gold. They each carry Bows, and they move as a unit in search of Zombies.
They kill normal Zombies, Husks and Drowned, but when they encounter a Zombie Villager, they dismount, throw a Spash Potion of Slowness II at them, and feed them a Golden Apple to cure them. The player can trade with them for Golden items, Splash Potions of Slowness, and Golden Apples, albeit at high prices. They also buy Rotten Flesh at lower prices than Clerics, so poor players can get some use out of them.
This serves as a cool background event, and for newer players, a tutorial on curing Zombie Villagers.