Doom 2 Extreme is a total conversion made by Chuck Lai. While weapons and monsters tweaks were done by him, 32 levels were created by others. Extreme gives larger hp-pool for both monsters and player, while boosting weapons to a whole new level. For example, automatic shotgun's fire rate is much faster than Doom's default chaingun.
Doom 2 Extreme was followed by Doom 2 Extreme Gold.