I picked up an item called Bloody Lust, and the effect in the lower right-hand corner was "Rage!". Plus, I get this totally rad headband.
I've tried getting hit and I've tried killing enemies, and once in a while, my damage will increase significantly and I'll turn all red.
How does Bloody Lust work?
For every enemy you kill, there is a chance that your damage will increase. That chance is high, but not 100% - I killed six flies and got four damage increases. It's also possible that each kill actually increases your damage, but the value that it is increased is not always enough to display another bar on the damage stat.
The damage increase is only in effect for the current room, though it is possible for the game to display an incorrectly-increased damage value on rooms after leaving the room.
