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!
Great mod for those looking for an extra challenge. One issue though is that some maps with very high monster counts can be extremely difficult to complete due to enemies increased damage. However, each death has always felt like it was my own fault, not unfairness of difficulty.