The phrase "rightful heir" refers to the person who has the legal and rightful claim to inherit something, such as a title, property, or position, usually passed down through generations. Full definition
Jarrod Emerson's Tribute to Alan Rickman Part 2: SENSE AND SENSIBILITY (1995) «John Dashwood» (James Fleet) made a promise to his ailing «Father» (Tom Wilkinson) that as rightful heir, he would make sure his stepmother «Mrs. Dashwood» (Gemma Jones) and her daughters «Elinor» (Emma Thompson), «Marianne» (Kate Winslet) and «Margaret» (Emille Francois) are financially stable. (ff2media.com)
Jesus is the son of The Living God, son meaning rightful heir to His kingdom (every other heir such as myself is adpoted through Jesus's finished work on the cross), The Holy God has only proclaimed one to be His beloved Son and this message was heard by many people. (religion.blogs.cnn.com)
Shrek no longer wants to be a king, and decides to track down the kingdom's other rightful heir, Artie (voiced by an awkward Justin Timberlake). (combustiblecelluloid.com)