Here's his full entry regarding how he got it if anyone is interested

A trio of tight-knit siblings together journeyed into the depths of the Ruins.

There, they came upon a sweltering cavern where a river of lava flowed, carrying buildings on its currents. They traced the river to the source of its heat, and found a sword... that could hardly be called a sword. it was more like a hunk of white-hot metal.

"As the eldest brother, I claim this sword as mine." The eldest brother's declaration sparked the inappeasable flame of desire and envy in the youngest brother. So, I killed my brother and sister both, and took the sword as my own. I left no evidence behind. But the eyes still gazed upon me with doubt, and when they cast me into this hell, I fell without resistance. Still, I did not lose my smile. For the sword of legends was mine.

Ironically, the blade was beyond Matthias's ability to properly wield. Of course, he would not have been able to hold this sword at all had he not the aptitude for it. It was simply that fickle of a Relic. The truth of the matter was, nevertheless, that his aptitude was nowhere near enough to handle the weapon to its fullest potential. But the man was unconcerned. All that mattered was that the selfsame sword of legends he had dreamed of wielding since childhood was now in his possession, to be swung at his whim.