Who said "The strength of a king is measured by his mercy."?

This quote is attributed to Rhaegar Targaryen.

The strength of a king is measured by his mercy. - Rhaegar Targaryen

"The strength of a king is measured by his mercy."

Rhaegar Targaryen Strength

"The dragon has three heads."

"I have a duty to my realm and to the future."

"Music is the voice of the soul."

"Love is the most powerful force in the world."

"Strength comes from within."

"A woman should be proud of who she is."

"A strong woman can overcome anything."

"The only thing I'm good at is killing."