I'll try to remember this right... The translation I read was a bit rough, but from what I gathered, after defeating the Celestial Court and unifying the realms, the Void Emperor, Ling Xiao, faces the final truth: the world he's conquered is itself a sealed prison created by higher beings to contain a primordial chaos. He's essentially the warden now. Instead of breaking the seal and risking everything, he chooses to maintain the balance from within, becoming the new 'lock' on the prison.
It's a bittersweet kind of ascension. He gets ultimate power, but it comes with eternal solitude and responsibility. He isolates himself in the void between realms, watching over creation. His love interest, the Moon Goddess Qingluan, can't join him there, so they have this tragic, forever-separated dynamic. The last chapter is just a quiet scene of him gazing at the myriad worlds from his silent throne, with a single line about how he finally understood the weight of the void in his title.
Some readers hated it for being too open-ended and bleak, but I thought it fit the novel's themes of sacrifice and the loneliness of power. It's not a happy 'riding into the sunset' ending, that's for sure.