The Elder Scrolls III: Morrowind

Moon sugar can be thought of as a substance similar to the real world's cane sugar with magical and addictive properties. In Morrowind and in other parts of the Empire the substance is illegal because it is classified as a narcotic - moon sugar is refined into the potent and highly-addictive drug known as skooma. In the game, possession of either skooma or moon sugar will cause most shopkeepers to refuse business with you until they are removed from your inventory. Khajiits are a notable exception since moon sugar is native to their home province of Elsweyr and is used as a spice there.

There are a few loose pieces of moon sugar in the game world that you can pick up, but major and larger sources of the substance can be found as random loot in containers located in abandoned/sunken ships and bandit/smuggler caves.