The sixth century was the last century in which Aksum acted as a great power. Kaleb, the father of Gebre Meskel, sent an army across the Red Sea into Himyar in southern Arabia in the 520s, an intervention presented in Christian sources as the punishment of a Jewish king who had persecuted the Christians of Najran, and an event that drew Aksum, Rome and Persia into the same theatre. Kaleb is said in Ethiopian tradition to have abdicated afterwards and retired to a monastery, sending his crown to Jerusalem.
What came next is poorly recorded. Aksum minted coins into the seventh century, but the sequence of rulers cannot be reconstructed with confidence, and there is no Aksumite king list, no annals and no dated document for the period. The rise of Islam removed Aksumite trade from the Red Sea, the port at Adulis declined, the coinage ended, and the political centre moved south into the highlands over the following centuries. Ethiopian Christianity survived that transition intact and largely alone, cut off from the Mediterranean churches for long stretches, which is a substantial part of why it looks the way it does.
Gebre Meskel sits at the hinge. Tradition assigns him a reign of considerable length, credits him with churches and with the patronage of Yared, and pairs him with the monastic founders of the same period, the group remembered as the Nine Saints who established monasteries and translated scripture into Geez. Whether the pairing is chronological or thematic is not recoverable from the sources. The atlas records his reign dates as unknown rather than as debated, because there is no scholarly position to weigh: there is a gap.