The primordial Greek deity who personified darkness and was Nyx’s brother was Erebus.
In Hesiod’s Theogony, Erebus emerged from Chaos alongside Nyx, the personification of night. The two primordial beings were siblings and consorts, and their union produced Aether, the bright upper air, and Hemera, the personification of day. This pairing creates a mythic contrast: darkness and night give rise to the luminous regions and cycle of daytime.
Erebus was not an Olympian god such as Zeus or Athena. He belonged to the earliest generation of cosmic powers, before the Titans and Olympians came to dominate Greek mythology. The name also referred to the darkness of the underworld, or to the shadowy region through which the dead passed on their way to Hades. That overlapping use is why Erebus can mean both a deity and a place.
The alternatives are related primordial figures but do not fit the clue. Aether represented upper brightness, Chaos was the initial cosmic gap or expanse, and Tartarus was both a primordial deity and a deep underworld abyss. Erebus is therefore the specific personification of darkness in this family group.