The One who is the Greatest Night.
The One who embodies the darkness of the night.
She is the cosmic night or the time of total dissolution, when all creation is absorbed back into the infinite, returning to its primordial state.
It is the time when the universe enters into the state of deep rest, signalling the end of a cosmic cycle.
She is that Great Night, where the old is destructed to pave the way for the new.
It is the phase of spiritual renewal.
This is the time of introspection and transformation, leading to a bright dawn, a new beginning, of higher consciousness.
On an internal level, this could be when a Sadhak transcends his limitations, begins on a new path or practice, or even when waits for a breakthrough in his sadhana.
Hence the name, Maharatrih.