Fernando IV was something of a nonentity, a man of little ability and
          weak character. His sobriquet of 'the Summoned' refers to the
          circumstances of his death. Two brothers were accused of the murder of
          one of the King's gentlemen and, although the evidence against them
          was slight, Fernando IV ordered their execution without trial. As they
          went to their deaths protesting their innocence, they summoned the
          King to appear before God's judgment seat within thirty days.
          Fernando, who was only twenty-six, paid little heed to this; however,
          on the thirtieth day after the incident, he lay down in the afternoon
          for a short rest and was found dead by his attendants when they became
          alarmed by his prolonged absence and went to investigate
 

Source: Leo van de Pas