Mondi Morbidi (morbid/soft worlds) explores the subtle yet powerful influence of the world builder in immersive video game environments. Thanks to advancements in computing power and game engine technology, contemporary game worlds are usually built procedurally through code and modelization. This approach implies that the human is always in control of our in-game experience, influenced by various biases and other human-built tools that are inadvertently passed on to the players within the game world. This project aims to alter this paradigm in order to question the notion of human control in video games, sparking a discussion on the delicate process of building game worlds and the influence this has on our in-game agency. In Mondi Morbidi, the game worlds on display result from a collaboration between a human and a physarum polycephalum - an acellular slime mold or myxomycete - popularly known as "the blob". This collaboration seeks to ask the question: What if we could create symbiotic game worlds that both depend on us and respond in an organic way by making decisions of their own?