The eighty-fifth ruler of Ashur was Mutakkil-Nusku, son of Aššur-dān I. According to the Assyrian King List [/riao/kinglists/assyriankinglist/index.html] (AKL), he became king after exiling his brother Ninurta-tukultī-Aššur to Babylonia. There is some confusion/uncertainty in the sources regarding the length of time that Aššur-dān I and his sons Ninurta-tukultī-Aššur and Mutakkil-Nusku ruled over Assyria. The Korsabad and SDAS copies of the AKL record that Aššur-dān exercised kingship for forty-six years, while the Nassouhi version of that text states that he reigned only twenty-six years. Both of his sons are said to have held authority for ṭuppīšu (lit. "his tablet"), a term whose precise chronological meaning is still uncertain. It has been suggested that this scribal notation indicated that the periods that Ninurta-tukultī-Aššur and Mutakkil-Nusku had exercised authority had been subsumed under the reign of their father Aššur-dān. Thus, the combined total for these three men was likely forty-six years.
It is clear from the AKL that relations between Ninurta-tukultī-Aššur and Mutakkil-Nusku soured over the course of their father's long reign. The two fought, Ninurta-tukultī-Aššur was driven into exile, and Mutakkil-Nusku seized the throne. Upon his death, Mutakkil-Nusku's son Aššur-rēša-iši I became king. Given the turbulent (and probably brief) period that he held authority, it comes as no surprise that not a single inscription of Mutakkil-Nusku has been discovered.
Poppy Tushingham & Jamie Novotny
Poppy Tushingham & Jamie Novotny, 'Mutakkil-Nusku', RIA 2: Inscriptions of Adad-nārārī I to Aššur-rēša-iši I, Th RIA Project, 2023 [http://oracc.org/mutakkilnusku/]