Simplify the following text for a Grade 5 native English-speaking person.
Original: King Edward the Martyr or Eadweard II ( c. 962 â `` March 18 , 978\/979 ) followed his father Edgar as King of England in 975 , but was murdered after a reign of only a few years .
Simplified: Edward the Martyr ( Old English : Eadweard ) ( c. 962 â `` 18 March 978 ) was king of the English from 975 until he was murdered in 978 . Edward was the eldest son of King Edgar , but not his father 's acknowledged heir .