This is a solid black floor lenghth (maxi) dress/gown by RSVP by Nykaa Fashion in size S. It is one shoulder but has a golden chain on one side that can be tucked in if you want. There's extra fabric/ruched at the sides to give a flattering effect. Also has a sexy side slit.

I've worn it only once for a party.
Attached pic shows a similar dress by the same brand.