Devourers gain +3% hit points, +1% attack speed and +1% Biotic Leech per stack of Biomass. Can stack up to 100 Biomass. Passive: Heal 1% hit points per Biomass.
Essence Hoarder: Devourers can collect an additional 25 Biomass (summary 125).
The Limitless: Devourers can collect an additional 100 Biomass (summary 200), but gain +1.5% hit points, +0.5% attack speed and +0.5% Biotic Leech per stack.
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)
The devourer's model exists in the Galaxy Map Editor for StarCraft II: Heart of the Swarm. It remains an air-to-air unit, but would mutate directly from a larva instead of the mutalisk. It has the same corrosive acid effect as in the original game, splashing targets with acid spores that reduce the armor of units by 1 and their attack speed by 0.1 for each spore. The effect can stack up to 20 times. The devourer has three upgrades associated with it that are tentatively tied to the evolution pit:
Gaping Maw: Devourer's attack speed increases by 25%.
Improved Osmosis: Acid spore debuff lasts twice as long.
Prescient Spores: Devourer can attack ground units.
Devourers can be seen in glass tanks in the Umojan research facility in "Back in the Saddle."
Dustin Browder has said that it was not "cool" enough for him, and on the other hand there was little use for air-to-air units in the campaign.