Enemy Echelons

Enemy Echelons going forward is an API.  The Strong Mobs Below mod is the official replacement that uses this API.
Legacy MOD versions will still be available.

MODID: eechelonsapi

Enemy Echelons API will also have Server-Side versions available.

MODID: eechelonsapi_server

MOD ID: eechelons

Enemy Echelons is a mod that assigns levels and attribute modifications to mobs based on the world height range that they spawn at. This is a Vanilla+ mob progression mod in the same vein as Champions and Sliding Health.

The goal was to create a mod in which the mobs progressively get more difficult the farther you travel down in the Overworld. It can be customized to be more difficult the farther you travel up as well, or both. I believe it works best in dimensions where there is a clear progression/change in the terrain, but it can be customized to any dimension. 

Echelons

An echelon is made of all the mob attribute modifiers for a custom defined set of dimensions and mobs. Each echelon definition has a list of stratum defined by a min and max y-value. Each strata has a list of level-weight entries (histogram). These define the likelihood of a level being assigned to a mob upon spawn. 
For more info and examples, see the Echelons wiki page.

Difficulty Level

A difficulty level or difficulty is assigned to a mob at spawn time and this difficulty determines the amount of modification to its attributes (health, damage, speed, etc).

Configuration

Enemy Echelons is very configurable, allowing you to add as few or as many levels as you wish and to setup their respective attribute modifiers. See the Configurations wiki page.