Gathering the Vorcha that littered the terminus systems the Reapers re-purposed them as Special Reaper infantry. Fast and agile on the battle field they climb walls and can leap great distances to get at their prey. When attacking they spit a highly corrosive goo that melts armor and flesh. When sustaining damage they bleed their corrosive goo making them dangerous in close-quarters combat.
That... is arguably the coolest, creepyest and most disturbing thing connected to the mass effect universe I have ever seen.... good lord I would not enjoy having to fight against these things in single or multiplayer.