Achlys

In Greek mythology, Achlys (Greek: Ἀχλύς, Akhlys, "mist") was described as the "eternal Night" and as a primordial goddess who existed even before Chaos. According to Hesiod, she is the personification of misery and sadness. She also may have been the goddess of lethal poisons according to Nonnus.

Other sources claim her to be a daughter of Nyx and, thus, number her amongst the Keres.

Hesiod
Hesiod describes Achlys as being depicted on the shield of Heracles as:

And beside them [the Keres (Deaths) and the Moirai (Fates) on the battlefield] was standing Akhlys (Achlys), dismal and dejected, green and pale, dirty-dry, fallen in on herself with hunger, knee-swollen, and the nails were grown long on her hands, and from her nostrils the drip kept running, and off her cheeks the blood dribbled to the ground, and she stood there, grinning forever, and the dust that had gathered lay in heaps on her shoulders was muddy with tears.

Nonnus
The possibility of Achlys being a goddess of deathly poisons comes from Nonnus, who writes:

''(Hera spies the nurses of the infant god Dionysos:) Hera, who turns her all-seeing eye to every place, saw from on high the everchanging shape of Lyaios [Dionysos], and knew all. Then she was angry with the guardians of Bromios. She procured from Thessalian Akhlys (Achlys, Death-Mist) treacherous flowers of the field, and shed a sleep of enchantment over their heads; she distilled poisoned drugs over their hair, she smeared a subtle magical ointment over their faces, and changed their earlier human shape. Then they took the form of a creature with long ears, and a horse's tail sticking out straight from the loins and flogging the flanks of its shaggy-crested owner; from the temples cow's horns sprouted out, their eyes widened under the horned forehead, the hair ran across their heads in tuft, long white teeth grew out of their jaws, a strange kind of mane grew of itself, covering their necks with rough hair, and ran down from the loins to feet underneath.''