What is the story behind Ravana's boon from Brahma?

 

What is the story behind Ravana's boon from Brahma? What was the boon? How was Ravana defeated by Hanuman if the boon was that he couldn't be defeated by any Vanara or monkey?

The boon received by Ravana from Lord Brahma is described very briefly in Valmiki Ramayana. Valmiki Ramayana says Ravana performed an extremely difficult penance for ten thousand years to satisfy Lord Brahma.

That courageous one who in early times has practised ascesis for ten thousand years in a great-forest, and who on accomplishing it has dedicated ten of his heads to the Self-Created Brahma, she saw such a Ravana with such ten heads.

Source: Valmiki Ramayana Aranya Kanda Sarga 32

Finally after the end of ten thousand years, Lord Brahma appeared before Ravana and asked him the boon of his choice. Ravana asked for a boon that that he should not be killed by gandharvas, yakshas, gods or by demons. Ravana left out humans and vanaras assuming they were of no match for him.

When all the deities spoke to Brahma thus, he thought for a while and then said "Aha! The idea to kill that evil-minded Ravana is discerned.

"Ravana said while seeking boon that, 'I shall not be killed by gandharva-s, yaksha-s, or by gods, or by other demons...' and I also said 'so be it..."

Source: Valmiki Ramayana Bala Kanda Sarga 15

Ravana was defeated once by Lord Hanuman as the boon is about not getting killed only. Also the boon did not include Vanaras which makes it clear that Lord Hanuman can kill Ravana without violating the boon which Ravana received.