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!
This is a great mod. I love that this mod doesn't change the doom gameplay much, it just makes it harder. The mod doesn't bloat the game with a million different enemies and weapons to where it feels like a completely different game. This is definitely a mod to try if you are a veteran Doomer and want to replay some older megawads that have become too easy.