From Halopedia, the Halo wiki
Blightstalkers take their name from the Blightlands in which they primarily reside. They function as keymind coordinators in environments in which no intelligent hosts are available for assimilation, and serve as hunters which scour the interior of the blightlands for those who may have survived the first wave of Flood infection prior to the blightland's establishment. They are large and aggressive predators, typically hunched over to use their forelimbs for movement. While stalking the blightlands, Blightstalkers act as a command node for local Flood forces which - when combined with their own speed and power - makes them extremely dangerous foes.