the gloom of the grave, tomb or grave, death
穸 centers on the darkness and finality of the grave: the gloom of a tomb, death itself, and the burial place. The meanings all point to the somber, hidden space of the grave.
穸 combines 穴 (hole, cave) and 夕 (evening), suggesting the darkness of a burial pit or the gloom of evening associated with the grave. The exact historical development is uncertain, but the components visually reinforce the idea of a dark, enclosed space.
Picture a deep hole (穴) in the ground at evening (夕), growing dark and gloomy. This is the grave, a place of death and eternal darkness.
For セキ, imagine a grave so old that it's covered in dry, withered grass, like a 'seki' (セキ) patch of land. The sound 'seki' echoes the desolation of the tomb.