The best open source alternative to Diablo is Flare (game). If that doesn't suit you, our users have ranked more than 50 alternatives to Diablo and seven of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Diablo are FreedroidRPG, Erebus, Ryzom and Summoning Wars.
Flare (Free Libre Action Roleplaying Engine) is a simple game engine built to handle a very specific kind of game: single-player 2D action RPGs. Flare is not a reimplementation of an existing game or engine. It is a tribute to and exploration of the action RPG genre.
The game tells the story of a world destroyed by a conflict between robots and their human masters. Play as Tux in a quest to save the world from the murderous rebel bots who know no mercy. You get to choose which path you wish to follow, and freedom of choice is everywhere in...
Erebus is an Open Source RPG (Role-Playing Game), for PCs, smartphones, tablets and handhelds.
In Ryzom there are no classes to choose. Develop all the skills of your character harmoniously or choose to concentrate on one skill and become a master. You are not forced to choose a faction when you create your character. Join a faction during your journeys, or not.
Summoning Wars is an open source role-playing game, featuring both a single-player and a multiplayer mode for about 2 to 8 players.
Lips of Suna is a tongue-in-cheek action RPG. It's an open world game that focuses on action heavy and versatile gameplay, dynamically changing game world and ease of customization and modding.
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Flare is merely a game engine, but the devs' main mod used to demo it: The Empyrean Campaign - is definitely inspired by Diablo, feel-wise(even the music at the main menu is a pleasant similarity).