During Kerrigan's resurgence, Abathur discovered the vile strain by forcing roaches on the world of Mehlus IV to drink microbe-infested water on the planet's surface. The roaches that survived the toxic ingestion adapted to produce acidic saliva which were altered by the byproducts of the microbes. When the vile attacks a target, their acidic saliva will act as a constrictive agent. As newer layers are added with each attack, the old layers harden and tighten thus debilitating the target.
Its attacks slow normal enemy units' movement speed and attack speed by 75%. Heroic units instead suffer a penalty of 20% to both movement speed and attack speed.
The slowing attacks of the vile roach make it very effective against larger enemy units such as the colossus, thors, siege tanks and archons by making their already slow attacks even slower. With proper micro, vile roaches can also kite zealots easily. They are also useful against ultralisks and hybrids, greatly reducing the threat they pose. However, the slowing effect is somewhat wasted on masses of smaller units, such as marines.
When compared with the corpser, the vile is more effective against larger, heavier hitting units such as colossi and thors, while the corpser is more effective against smaller closely grouped enemies.
The Vile roach gain +3% hit points, +1% attack speed, +5% armor and +1% Biotic Leech per stack of Biomass. Can stack up to 100 Biomass. Passive: Heal 1% hit points per Biomass.
Essence Hoarder: Vile roaches can collect an additional 25 Biomass (summary 125).
The Limitless: Vile roaches can collect an additional 100 Biomass (summary 200), but gain +1.5% hit points, +0.5% attack speed, +2.5% armor and +0.5% Biotic Leech per stack of Biomass.
The amount of collected biomass vary, based on difficulty and the amount of supply consumed by the killed enemy unit:
Collect 5 biomass per supply (Casual and Normal)
Collect 4 biomass per supply (Hard)
Collect 2.5 biomass per supply (Brutal)