Baldr is son of Frigga and Odin.
Baldr- the handsome had been having nightmares. And he tried to his mommy Frigga.
And as any mommy, his mommy listed out everything that could harm her son and asked for oath that no one and no thing shall harm her son.
She being the ultimate mommy and wife of Odin and all, everything accepted and took the oath.
She felt that mistletoe was too young and fragile and didnt ask for mistletoe's oath.
He is told to be the most handsome of all.
Having acquired this boon, all the gods were using him like guinea pig to test the boon by shooting and trying all ways to kill him but he remained unharmed.
The mischevous and jealous Loki tricked Frigg to spill this loophole and used the blind brother Hoor, to shoot mistletoe at him and Baldr-only birth son of Frigga, fell head.
Human and Ravana:
Ravana is the King of Sri Lanka former Ceylon.
He wanted to become an immortal emperor. He performed an intense penance to Lord Brahma.
He kept chopping his head off as part of penance but like Hydra it kept growing but not in multiples just one for one.
At last, Lord Brahma, pleased with his austerity, appeared after his 10th decapitation and offered him a boon.
Since immortality was no longer in store, Ravana was given choice to pick anything other than immortality.
Ravana listed everyone -gods, heavenly spirits,demi-gods,monsters, serpents, beasts, every single thing and gave the list to Brahma and asked him to grant him the oath that he shall not be killed by anyone or anything in that list.
To Brahma this seemed easier than immortality, so he said "Wish granted".
Mortal men didnt seem to be a threat and he didnt include that.
Usually in hindu mythology after acquiring the boons these booned souls start to behave like goons and start to do unspeakable things.
Ravana did the same and in order to vanquish him, Lord Vishnu takes avatar of Rama(Human avatar) and killed him.
Moral of the story: If death is inevitable better enjoy and go for it. Why to hurt and trouble yourself to do the penance and get the boon and later get killed by trick and get into history with a bad title or as a bad guy. Live life of king size.