id Software is an American video game development company currently based in Richardson, Texas, and is the publisher for all games in the Heretic/Hexen series (with the exception of Heretic II and Portal of Praevus). It is most well-known for the creation of the Doom and Quake franchises, and arguably for pioneering the first-person shooter genre.
All games in the Heretic/Hexen series are based of of id's engines. Heretic and Hexen use the "id Tech 1" engine, and Hexen II and Heretic II use the "id Tech 2" engine (the latter being a heavily modified version).