Okay Sonu seems to have given a complete synopsis.
Dev is Childhood friend of Chandni and Dev's father is Mahant of the temple
Chandni's step mother plays tricks and makes Chandni believe that Dev's mother is a witch. Also she conspires with the jeweller and changes the temple jewellery to fake. This causes the mob to attack and kill both the parents of Dev.
Dev tries to run with his younger brother but he gets attacked by the mob and he has to throw off his baby brother from a moving train. His brother gets adopted into a muslim family and grows up to be Veer
Dev now has returned back as Advay and wants to revenge on everyone who caused destruction of his family. And he hold Chandni responsible because Chandni's word/statement of Dev's mother is flying caused the mob to burn his mother.
That being said Dev totally loved Chandni when he was growing up and he was her protector. So whenever Chandni is in distress and in problem Dev in Advay can't ignore her pleas and comes to save her.
In Short Dev is her protector whereas Advay is her tormentor and both are the same person. Advay knows who Chandni is but Chandni doesn't know that Advay is her childhood love Dev.
Dev is quite possessive of Chandni and hence can't allow Chandni to marry someone and go away. During the marriage phase also Advay might try to hurt Chandni whereas Dev will be her savior. Need to see how this marriage goes ahead. But the story synopsis is that Advay's hate gets overpowered by Chandni and Chandni heals Advay.
Both Chandni and Advay are broken and they need each other to heal and become complete. This is the gist of this show. Hope it helps.