Béla III was King of Hungary and Croatia between 1172 and 1196, however he had some difficulties getting recognition of his title.1.
Saint Ivan of Rila was born app. 876 a.c. He was a contemporary of the reign of emperor Boris I, his sons Vladimir (Rassate) and tsar Simeon I The Great and the son of the latter - tsar Peter I.
...Taking advantage of the emerging anarchy in the Byzantine Empire, Béla advanced as far as Niš and Sardica in the first half of 1183.2 In Sardica, he seized the casket containing the relics of Saint Ivan of Rila, and ordered it "to be transported with great honors to his land and to be laid down with honor in the church"3 of Esztergom, according to the saint's Life from the Sofia Prologue...
The relics were returned back after three years.
Why would the relics of a local saint be of any use to Béla III, could they be used as a form of legitimation of his title, having in mind that the Bulgarian and Hungarian dynasty blood lines at that time were connected?
Could Saint Ivan of Rila be heir of emperor Boris I?
1. Although the Hungarian prelates and lords unanimously proclaimed Béla king, Lucas, Archbishop of Esztergom opposed his coronation.
2. Curta 2006, pp. 334–335.
3. Life of John of Rila from the Stishen (Sofia) Prologue, p. 266.