Ungoliant was a truly gigantic spider god type creature who is an ancestor of Shelob, the spider that attacked Sam and Frodo. She wasn’t initially so huge, but Melkor fed her various powerful objects and she grew and grew. They stayed allies until she demanded to consume the Silmarils, but, like every other possessor of the Silmarils, he didn’t want to give them up, so they fought. I think that’s what’s being represented here.
By Ruben
Amazing art, she looks terrifying. It is worth mentioning Melkor promised to give them to her in exchange for her help. The fight was caused by his refusing to pay up after.