Constantine VI reigned under his mother Irene, pushed her aside, was pushed back, and was blinded on her orders in 797 in the room where he had been born.
Constantine VI became emperor at nine in 780 with his mother Irene as regent, and spent his entire adult life failing to get out from under her.
He was of age in 790 and forced her from power with army backing, then let her return as co-ruler within two years, which was the decisive mistake.
In 797, after a series of military failures and a scandalous second marriage had cost him his support, she had him seized and blinded in the room he had been born in. He disappears from the record afterwards.
He was born in 771, crowned co-emperor at five, and was nine when his father died. His mother governed as regent for a decade and had no intention of stopping.
A betrothal to Charlemagne's daughter Rotrud was arranged and then broken off, a piece of diplomacy that mattered more than it seems: a Frankish–Byzantine marriage would have changed the shape of the next century, and its failure contributed to Charlemagne's imperial coronation in 800.
Irene instead married him to Maria of Amnia, chosen through a bride-show, in 788. He resented the choice and eventually repudiated her, which became the crisis of his reign.
He is generally judged weak, and the judgement is largely fair, but it is worth being precise about what the weakness consisted of.
He was not incapable — he campaigned personally, held commands, and had enough support in the army in 790 to remove his mother from power outright. What he lacked was the ruthlessness to finish anything: having deposed Irene, he restored her; having gained the army's loyalty, he spent it by blinding and mutilating his uncles and having a popular general's eyes put out.
The sources for all this are monastic and hostile to him for the marriage rather than the politics, so the moral emphasis is skewed. But the pattern of half-measures is visible through the bias and it is what destroyed him.
His military record was poor: defeats by the Bulgars at Marcellae in 792 and by the Arabs, and a mutiny in the Armeniac theme that he suppressed with mutilations that cost him the army's goodwill.
The fatal issue was his marriage. In 795 he repudiated Maria, forced her into a convent, and married his mistress Theodote — an act the church condemned as adultery and which produced the "Moechian controversy", a schism between the emperor and the monastic party led by Theodore the Studite.
That gave Irene her opening. With the monks against him, the army alienated and his own record indefensible, she moved in August 797: he was seized in the palace and blinded, deliberately in the Porphyra, the chamber where imperial children were born and where he had been born himself.
He was the last male ruler of the Isaurian dynasty. Irene ruled alone for five years, was deposed in 802, and the family that had saved Constantinople in 718 ended in a palace coup.
The blinding is one of the most notorious acts in Byzantine history, and its notoriety is deserved — but it is also the logical endpoint of a system with no rule of succession, in which the difference between regent and ruler was whatever the parties could enforce.
His deposition had a distant consequence in the west. With a woman on the throne in Constantinople and no emperor the Franks recognised, the papacy crowned Charlemagne emperor in Rome on Christmas Day 800.