How Ravana Earned the Boon of Being Invincible?

Times back, the demon king of Lanka, There was an Asura called Ravana. He had  10 heads.

He was very powerful and knowledgeable. He often worshipped lord Siva.

Ravana  was a great scholar possessing extensive knowledge about all four Vedas and six Shastra.

Ravan wanted to be invincible. So he started praying to Lord Brahma. 

He did very long prayer to please lord Brahma. He was firm and passionate about his devotion. Ravana once performed an intense thapas  of Lord Brahma for several years. Ravana wants to get God's attention and ask him to fulfil his wishes. When he saw no results of his rigorous penance and failed to please Lord Brahma, he thought to use another way round to show his devotion and impress Lord Brahma

The demon King Ravana dipped in his passionate devotion started to cut his heads one by one and surrender them in the name of Lord Brahma. He has almost cut nine of his heads and about to cut his last head to offer Lord Brahma.

Just at the moment Ravana was about to cut the tenth and last head, Lord Brahma appeared & stopped him from cutting his last head. Lord Brahma was quite impressed by his sacrifice, offerings and passionate devotion. Then Lord Brahma asked Ravana to present his wishes in front of him.

Ravana asked to lord that he wanted to be immortal. then said that he wants to be immortal. Lord Brahma replied that he cannot fulfil his desire of being immortal but he can ask for any other wish if her wants. And so, the demon King Ravana then asked for immunity and invincibility from all Gods ( Devtas ), Serpents, Demons ( Raakshas ), heavenly spirits, wild beasts and transgender.

As he has egoistic behaviour, he forgot to include humans in his prayer or wish while asking this boon from Lord Brahma. Lord Brahma granted him this boon which led Ravana to believe completely that he was then absolutely invincible.