Harmony Patching

Harmony is a library for patching, replacing and decorating .NET and Mono methods during runtime. It ships with the game and gives modders a much greater degree of freedom, which however comes at the cost of reduced game update resistance and mod compatibility. Due do that fact it is recommended to avoid Harmony where possible, but sometimes it is the only way to make necessary code changes for a mod to work. If you need a new hook in the vanilla game, feel free to suggest one on github

Code samples for Vintage Story specifically are still missing here, but until then, you can refer to the official documentation