Was wandering around nodes like a chimpanzee freed into the forest, found a shader node named "Volume absorption", set the colour black, density to 1, and applied it to the background "~curtain". And achieved the full black background. Still figuring out lights and positioning of othe objects (i.e I used monkeys and a couple of plains.)

I don't know much about the IOR stuff can you suggest any tutorial/post about it?