Real-time-strategy ECS sandbox.
Forge is a game, mod API, and game engine rolled into one. It make be broken apart in the future, but for now, it makes the most sense to keep it bundled together.
Forge is described as a sandbox because it's essentially an RTS game with no theme or story/campaign. It uses an entity-component-system structure to allow for the creation of almost any unit imaginable. In Forge it's possible to pit stone age gatherers against aliens, or Roman legionaries against U.S. Marines.