Progressive Difficulty is a Minecraft mod for version 1.12 (with a partially supported 1.11 backport). The mod's goal is to provide a less flat difficulty curve, while being as configurable as possible for the user or modpack maker. The way this is achieved is though applying difficulty modifiers to mobs that spawn based on a set of configurable difficulty controllers. Through configuration, the mod could cause only charged creepers to spawn, all mobs to have speed or resistance, or allow players to play in peaceful mode until certain conditions are met. You can allow a slow build of difficulty, day over day, until the world is filled with invincible zombies, or make them only spawn in the depths of cave systems.
Because the mod is so configurable, you should check out the wiki for information on what all is configurable, and how.