Halo 3's Flood overruns 7 Days to Die: combat forms, Blood Moon infection swarms and Halo CE's Flood music, read from your own MCC.
The Flood has found Navezgane.
You need both games. Nothing from Halo is included: the Flood models, skeletons and textures are read when the game starts from your own Halo: The Master Chief Collection (halo3/maps), and the music streams from your own Halo CE sound bank. Melty starts 7 Days to Die without EasyAntiCheat so the mod can load. Single player.
Halo is owned by Microsoft / 343 Industries and 7 Days to Die by The Fun Pimps; this fan mashup isn't affiliated with either. Thanks to the Halo modding community (Reclaimer, Assembly) for documenting the map formats.