The eighty-fifth ruler of Ashur was Mutakkil-Nusku, son of Aššur-dān I [http://oracc.museum.upenn.edu/riao/thekingdomofassyria13631115bc/ashurdani/index.html]. According to the Assyrian King List [http://oracc.museum.upenn.edu/riao/kinglists/assyriankinglist/index.html] (AKL), he became king after exiling his brother Ninurta-tukultī-Aššur [http://oracc.museum.upenn.edu/riao/thekingdomofassyria13631115bc/ninurtatukultiashur/index.html] 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 [http://oracc.museum.upenn.edu/riao/thekingdomofassyria13631115bc/ashurreshaishii/index.html] 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', The Royal Inscriptions of Assyria online (RIAo) Project, The RIAo Project, a sub-project of MOCCI, 2017 [http://oracc.museum.upenn.edu/riao/thekingdomofassyria13631115bc/mutakkilnusku/]