... is Nephthys, the sister of Isis, goddess of the hidden, of the unmanifest and of darkness as opposed to the manifest and the light. Because she represents interior structure, order and secret knowledge, one of her titles is "Lady of Buildings." A building has rooms, and rooms have walls; Nephthys represents limitation. It is quite significant that she is the wife of Set, who is in turn enemy of Osiris, the husband and brother of Isis. But Isis and Nephthys, the sister of light and the sister of darkness, make a dyad -- both required for the reconstitution of Osiris. They are the apparent and the concealed... (Ya zahir, ya batin, say the sufis) -- the manifest reality and the quantum process, the law of vibration, within it. Nephthys is the invisible, Isis is the visible.

Image of Nephthys (public domain, wikipedia). Note the head-dress. Isis wear a throne on her head -- signifying rising power, rulership. Nepththys has the same throne upside down, and the bowl is equally the part of the sun that has already sunk below the horizon at sunset. She is Isis inverted, hence her husband is the enemy of Osiris.