Choose a damage type after each long rest. Until your next long rest, the class features granted by Way of the Purifier at 3rd level, 6th level, 11th level, and 17th level work exclusively against enemies that have an innate resistance/immunity to the damage type you've chosen.

Rage of the Pure

Starting at 3rd level, your hatred of certain entities fuels you with an intense determination. When attacking an enemy that has innate resistance or immunity to your chosen element, double your proficiency modifier for your unarmed strikes' hit modifier. In addition, you can choose to spend Ki points, adding an extra 2d4 to your damage roll for every Ki point used. If the attack misses, refund all Ki points spent.

Ki Purification

At 6th level, you gain the ability to steal your enemies' Ki energy. When you use an action, bonus action, or both to attack a creature with innate resistance or immunity to your chosen damage type, you can choose to heal half the damage you deal to that creature on that turn. This ability becomes usable again after a long rest.

Superior Resistance

Beginning at 11th level, you gain resistance to the damage type you chose after every long rest. This resistance resets after every long rest.

Strike of Damnation

At 17th level, you gain immense planar power over the Ki of your enemies. When attacking a creature with innate resistance or immunity to your damage type with an unarmed strike, you can choose to spend 5 Ki points, forcing the creature to roll a DC17 Wisdom saving throw. On a failed save, the creature is banished to their native plane of existence. If the creature is already on its' plane of existence when you use this feature, it becomes banished to a harmless demiplane for 1d4 hours.