"The bile crawlers are commonly spoken of among the peasantry in Khanduras, most often invoked to scare unruly children into their beds. Fables hold that the crawlers are the half-formed demonspawn who hatched before their time. They resemble nothing more than a bloody, ravening mess and possess a voracious appetite to human flesh."— Abd al-Hazir(src)
Bile Crawlers usually hide within the growths of demonic corruption in the High Heavens, pouring forth from it when the cluster is destroyed. They appear one by one (so it's hard to hit them all at once), 8-10 per cluster. Some are also found in Hell Rifts. They are significantly stronger and just as fast, but less numerous than Shadow Vermin.
Once the players bypass the Gardens of Hope, the Bile Crawlers are no longer encountered.