Ryzom also known as The Saga of Ryzom is a free and open source massively multiplayer online role-playing game (MMORPG) developed by Nevrax for Microsoft Windows OS X and Linux. Ryzom features open world gameplay. Instead of character classes it has a unique system for character abilities that allows players to construct custom actions like spells or attacks in detail. The game world is complex modeled incorporating realistic effects like changing weather and seasons. Computer controlled entities ('mobs') display complex behaviors and animals will for example graze in herds or hunt in packs. Ryzom offers a scenario editor that lets players design their own areas that can be uploaded and played on the official server.