Basic Knowledge

In singleplayer the mod is straightforward: each Wither killed and Ender Dragon (re)spawned will count towards a point of Difficulty, making them harder the next time they're spawned.
Each point of difficulty increases the Bosses stats and unlocks abilities.
In multiplayer the Difficulty of all the players near the Wither or the Ender Dragon is taken into account. The difficulty for the bosses is then an average of the sum of players' difficulties plus a bonus per player that's fighting the boss.

Modules and Features

The whole mod is split into three Modules (Wither, Ender Dragon and Elder Guardian), each Module is split in highly configurable Features. Each Module / Feature can be disabled individually.

Command

The mod adds a command /progressivebosses that lets you get, set and add to the boss difficulty per player. It also allows you to summon the bosses minions.
Syntax (<> is mandatory while [] means optional):
/progressivebosses <difficulty/summon>
/progressivebosses <difficulty> <targetPlayer> <get> [wither/dragon]
/progressivebosses <difficulty> <targetPlayer> <set/add> <wither/dragon> <amount>
/progressivebosses <summon> <entity> [difficulty]