This is a randomiser mod which changes a boatload of stuff but keeps to the original vanilla-like gameplay.
Some info on it:
-Each monster has typically 3/4 tiers where a random tougher monster will spawn in its place.
-Armor and Spheres have a random chance to spawn a stronger type/powerup.
-Weapons have been redone and feature reloading.
-There is an inventory system to make use of items/powerups which are randomly dropped by monsters.
-You can toss grenades and mines with weapons equipped.
There's plenty more in it but its been out for so long players probably know what its all about.
Spoiler: Latest release changes (v27b2)You can download it HERE.
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Spoiler: Gameplay videos