Hard Doom is a replacement/enhancement mod that replaces all monsters in Doom 2 with versions that are similar in role, but more dangerous. In general, Hard Doom is geared towards players who want a more difficult, genuine survival experience, and it has a variety of visual effects that are more subtle and don't block your view of the action. Over the years since its initial release, it has been meticulously balanced and rebalanced, with new features and effects added to make this a great mod to play through the popular megawads with, without changing Doom's flavor or core gameplay. This mod requires ZDoom, GZDoom, or Zandronum in order to play!
A small Hard Doom addon so the Super Shotgun drops in Ultimate Doom (and Doom II). This makes the mod easier and I suggest trying without it first. This is for version 7.1 of Hard Doom.