Nyx

Nyx is the widow of Erebus, Queen of Tartarus.
Now matriarch of The House Of Erebus.
She is the mother of Apate, Momus, Mercy, Nemesis and others.
Nyx first appeared in Season 2.
Nyx never really engaged in the hostilities Erebus initiated, instead behaving in a civilized manner to all she encountered.
At the end of Season 2 she was left as sole ruler of Tartarus.