Quake (1996) introduced polygonal characters and levels as opposed to sprites, as well as the first major mod kit available to consumers. The Quake engine is likely the reason 3D shooters took over on PC after the days of Duke and Doom. From there you had various builds of the engine and then Unreal, which gave players the Unreal Editor, a fully-fledged toolbox of level design, allowing everything from building a map to special AI triggers and events players could build into it in a non-coded, streamlined manner.