This is a Corrupted Mod, obtainable from Dragon Vaults in the Orokin Derelict.
This mod combined with Point Blank and Blaze give off a total damage increase of +240%.
With Primed Point Blank instead of Point Blank this adds up for a total damage increase of +315%.
In terms of crowd control, combining this mod and Hell's Chamber gives off impressive results, damaging a wide spread of enemies in every shot more effectively than shooting one target at a time.
Unlike Heavy Caliber, the rifle equivalent of this mod, Spread can be reduced by the Tainted Shell mod – though with a very high fire rate penalty, as cancelling out the spread increase from a maxed Vicious Spread would require a fire-rate reduction of at least 54%. This loss could be compensated for using a maxed-out Accelerated Blast or Rank 3 (or higher) Shotgun Spazz.
Alternatively, Narrow Barrel can reduce the accuracy penalty by half after hitting an enemy.
Convectrix and Phage are partially immune to spread penalty in their focused modes.