In Sat epi Dev tells he has only time till Durga's marriage to find his Nithya. It was really symbolic and true too. Because if Durga marries Shaurya Nithya will cease to exist.
Nithya and Durga. Two aspects of same Goddess. Both define Power. One the constructive power and the other destructive power. Nithya is that Shakti which is evoked with the mantra " Sarva Mangala Mangalye" in every auspicious occasion. Durga is the Shakti in front of whom people do penance so that her anger and wrath does not fall on them or their family.
Now from EHT perspective Payal believes her sister Nithya to be her protector. Even after suffering so much the girl calls out Nithya di Nithya di. She knows her sister will fight for her. To Payal Nithya means love and security.
Dev exists. He is a non entity. He has a powerful surname but no use of it. A man with immense potential with the right talent, right set of values if channeled correctly can transform both his family as well as the society. Unfortunately chained by the dogma of his mother who just does not want to let go of him to reach newer heights lest that may endanger him he lives a life in shadow. He believes Nithya to be his strength. Even while the world believe her dead he believes she exists and is always with him.
Now this Nithya which both Payal and Dev wants is transforming. She started the revenge with small small manipulations and in every case the victim of her manipulation ended up in police custody or we can say in the hands of law. But now the woman has started taking law in her own hands. The woman who called up goons to beat up Shaurya( I enjoyed every part of that epi. Watched repeat too.) and cripple him is slowly losing the Nithya in her. If calling up goons and beating up Shaurya is the beginning of her true revenge, what will be her true plans. Anyways they are going to be dark, very dark and if that marriage with Shaurya happens she will become Durga, Kali or Chandika in its most fierce form who will destroy the ground she is standing even if that means destroying herself. After a certain point there will be no difference between her and Sakshi in ruthlessness. Sakshi herself calls Durga her mirror image.
But that cannot happen. Between Sakshi and Durga there has to be difference. And that difference is the Nithya in Durga. Maa Durga is worshipped because Nithya, Gowri or Parvathi exist in her. It is because she knows the difference between good and bad. Her benevolent form is made visible to her true devotees and only the sinners see her fierce form.
Hence Nithya cannot die. Nithya needs to exist in Durga. The constructive and destructive powers have to co exist and destruction has to be balanced with construction too. Power should not be in wrong hands, it needs to be in the right hands who respect it and knows the importance of it.
If Nithya is not found before Durga's marriage to Shaurya then we can say it is darkness as there is no balancing power to stop the destruction. Hence Nithya has to be found before that, that soul has to be re-established in Durga for her own good and also for the good of people who love and care for her.