Basically, it's Doom with gratuitous, over-the-top amounts of blood, gibs, new animations for monsters, new weapon sprites, and a ridiculous amount of attention paid to making the gameplay very awesome. I was never one for the original Doom, but holy crap this mod makes it enjoyable. Also, there are fatalities... You'll just have to see that part for yourself. You can even flip the enemies off.
You'll need either the Skulltag port or a zDoom port(GZDoom works best performance-wise it seems), along with the doom2.wad from the original game.
You can get the mod here: http://www.moddb.com/mods/brutal-doom