The great grandfather of Hrothgar was Skiold also known as Scyld and he was the pioneer King in the Danes. He is well known as the little baby that was placed in a boat, and in his burial, he was decorated with many precious stones and also laid in the sea to be taken far off.
He gave birth to Frode who succeeded his throne and who was known for his role in the overthrow of Tunni. Frode gave birth to Halfdan who defeated his brothers to inherit the throne. Halfdan gave birth to Hrothgar who was aged and in his 50's when Grendel started terrorizing his kingdom.
