Sulochana was beloved wife of Meghnad, son of Ravana and Mandodari. She was a Naga Princess and was heavenly beautiful. Some retellings calls her daughter of Sheshnaga (who reincarnated as Laxmana). When Meghnad was killed by Laxman, he brought his severed head and gave it to Shri Ram. His hands however flew to Sulochana and wrote a letter with his blood to take his severed head from Shri Ram. Sulochana after taking permission from Ravana and Mandodari went to Shri Ram in a palanquin but she was decked up with gems and jewels. No one understood her arrival except Shri Ram and he was horrified when he knew the real reason why she wanted his severed head. She wanted to self immolate with Meghnad’s head. Shri Ram out of shock and horror told her that he would make Meghnad alive again if she left her vow of Self Immolation which shocked Vanara Sena. Shri Ram wanted to save Sulochana because she wasn't ready to remove her vow. In the end due to Sulochana’s persistence and repeated pleadings, he gave Meghnad's head with a sorrowful eyes and heavy heart. Sulochana took the head and cried and then self immolated herself. Her sacrifice gave pain to both warring sides because she was innocent in all this. Shri Ram was even ready to revive Meghnad but she didn't wanna betray Ravana and her martyred husband.
This story is not mentioned in Valmiki Ramayana but in Ananda Ramayana which is a much, much later source.
Source- Ananda Ramayana