[2] It is uncertain and perhaps doubtful that Tudḫaliya the Younger reigned as great king of Ḫatti.
The main source of information on Tudḫaliya the Younger are the Plague Prayers of Šuppiluliuma I's son Muršili II: With the Plague Prayers virtually our only source of information, Tudḫaliya the Younger remains obscure.
Based on the existence of seal impressions of Šuppiluliuma naming him the son of Tudḫaliya III, scholars have traditionally assumed that Šuppiluluiuma gained the throne by murdering his brother or half-brother Tudḫaliya the Younger.
[5] However, the context seems to exclude a sibling relationship between Tudḫaliya the Younger and Šuppiluliuma, and the discovery of the seal of Šuppiluliuma's first wife Ḫenti, naming her “great queen and daughter of the great king, the hero,” has led to the conclusion that Šuppiluliuma was a son-in-law (and at best adopted son) of Tudḫaliya III and therefore brother-in-law of Tudḫaliya the Younger.
A detailed and annotated genealogy of Hittite New Kingdom monarchs and their families, as reconstructed by Jacques Freu in his multi-volume work Les Hittites et leur histoire, presented as an alternative to the less detailed and sometimes differing reconstruction based on Trevor Bryce, The Kingdom of the Hittites.