Etterna is a rhythm game based on the StepMania engine that is focused on better accomodating the four-key keyboard metagame. Some new features include automatic difficulty calculation, a new scoring system, and engine optimizations. Etterna is generally compatible with content for StepMania 5.0. However, some StepMania 5.0 themes and noteskins may require modifications in order to function properly.
StepMania is software that was originally created in 2001 to serve as a Dance Dance Revolution simulator, during its many years of development the software grew to support even more game modes such as Pump It Up and Beatmania. The single most common game type is the original DDR style (4 keys). This game mode can be played on a Dance Pad or keyboard (or anything you can map 4 keys to, such as a Guitar Hero controller or a Gamepad). Over the years, the keyboard community for the game has grown extensively and broken barriers in relation to what Charts are possible to clear; Etterna is a community developed version of StepMania 5 specifically made with keyboard players in mind.
Etterna originally started as a mod for StepMania 5.0.12 that focused on improving performance. Later on it was merged with Mina's scoring system & difficulty calculator and added new features such as favorite files, song searching, and filtering. The project have sufficiently diverged from the original goals of StepMania as a program and became it's own fork with designated features for keyboard players.
The first public build of Etterna was available on December 4th, 2016. The project moved to its own repository on September 8, 2017 with the release of version 0.55.
The official repository for Etterna can be found here and the latest builds can be obtained from here.
For details on how to install the game, refer to the Installation guide.