prehistoric relics

七百年ほど前の嵐の日一度雷光を発し、それ以降この世界に魔物がはびこるようになったという。

百年ほど前にも一度光ったとか光らないとかいう話があるが、真偽不明である。

which, according to google translate, is

It is said that about 700 years ago, during a storm, a flash of lightning emitted once, and ever since then, this world has been infested with monsters.

There is a story about a hundred years ago that it once glowed and sometimes it never glowed, but the truth is unknown.

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transcript

two people on horseback in a shallow lake; staring at a large unknown metal device wreathed in moss and vines; with trees growing from the joints