Complex sentence: Some modern scholars ascribe the story of the "Oath of Heraclius" to the realm of legend doubting that Heraclius every made such a promise, instead ascribing this as a product of later apologists.
Split into simpler sentences: Some modern scholars ascribe the story of the "Oath of Heraclius" to the realm of legend doubting that Heraclius every made such a promise. Instead ascribing this as a product of later apologists.