For the grand strategy game Hearts of Iron IV, not only are official expansions released, but also various fan-made modifications. Sometimes they expand and transform the game, and sometimes they offer something else entirely.
Recently, a modder known as Yard1 introduced a mod that allows players to play the legendary shooter DOOM inside Hearts of Iron IV. In the current version, players will have access to the full first episode of DOOM (using the shareware version), as well as the first map from Freedoom.
The modder clarified that he "implemented the entire DOOM engine (based on doomgeneric)," so technically players will be able to run any WAD files compatible with doomgeneric.
However, he warned about a technical nuance: keyboard input in Hearts of Iron IV "works unstably, but the game can still be fully played."
The modification, aptly named "DOOM," was published by Yard1 in the "Steam Workshop" on the Hearts of Iron IV page – to download the mod, players simply need to "subscribe" to it.
Yard1 also presented the source code for the "doomgeneric" project with changes in his GitHub repository.