Flare is an open source, 2D action RPG licensed under the GPL3 license. Its game play can be likened to the games in the Diablo series.
While the core engine is written in C++ with SDL2, there is an easy to use modding interface. Using the modding interface, it’s possible to create entirely new games. On the Mods page, you can find games made by the Flare team, as well a few select community projects.