The 147th chapter? That's from the final stretch of the main story, right after the Monarchs' War on the Kamish battlefield. The 'important event' I always think of is Ashborn, the real Shadow Monarch, finally stepping aside completely. He says his goodbyes to Sung Jinwoo, releasing his full authority over the shadows. It's less a flashy battle and more this quiet, heavy transfer of power.
Jinwoo fully inherits the title and becomes the true, sole Monarch. This is the whole point the series title's been building toward, that 'Solo' part finally locking into place. What stuck with me was the visual of the two of them on that throne—Ashborn fading, and Jinwoo left alone with the absolute responsibility. It felt like the end of a mentorship and the start of a much lonelier reign.
You can feel the narrative gears shifting toward the final confrontation with the Rulers.