Munkar and Nakir aren't a myth, but according to mouslems creed are the two angels responsible of questioning the dead people in their grave..
their shape is horrible and aweful according to mouslems creed..
they ask the dead person 3 questions:
1- who's your God?
2-what's your religion?
3-what are you saying about your prophet(Muhammed)?
according to mouslems creed, the true mouslem will answer , while the disbelievers will not so the two angels will hit with a huge hammer as a kind of torture till the judgement day.