Heartbound ARG
The Heartbound ARG (Alternate Reality Game) is a companion game to Heartbound, and is freely available at Heartbound's official website.
Owning Heartbound is not required to play the ARG, however you will be unable to progress past a certain point without a walkthrough if you do not own the game, and the storyline of the ARG will not make any sense to you if you haven't played Heartbound.
Unlike Heartbound itself, this game is not an RPG, but rather a puzzle game, focusing on cryptograms, riddles and various other logic tricks. Also much unlike Heartbound, the game does not use the GameMaker engine, but rather is coded entirely from scratch in pure HTML. Due to the web-based nature of the game, it cannot be reasonably played offline, as every solution to a puzzle will lead the player to a new webpage.
The ARG is currently up to 7 chapters total, and can currently be found here: https://heartbound.fandom.com/wiki/ARG:Overview