This is the prophecy Ulliel is guarding.
It should have been kept a secret but since the vision came out on the old king's birthday where all the rulers of all realms were celebrating - pretty much everybody knows what's written in the prophecy.
The son of the king who reigned
for forty years and fifteen days
shall father the new king that shall seize
the five of the six realms.
His reign will be revered,
his shadow will be feared,
his followers will be rewarded
and foes obliterated.
For he shall be called The Son of The Son
and his name will forever live in glory.
The king who died reigned for forty years and fifteen days and his son, Henry, is trying to fill the prophecy and provide the heir that would be named The Son of The Son.
That's why he's taken the maidens as his lovers and waits to be given a son before marrying and becoming the new king.
Of course, the rulers of other reigns are not that thrilled that they will be conquered by The Son of The Son and their goal is to stop the prophecy from becoming true so as the king died after forty years and fifteen days they would start gathering an army and forming alliances to bring the Pursenna Realm down.
But the Prince isn't that dumb nor are his protectors so all are obliged to stay in the palace keeping king's death a secret until the heir is born.