Backstabbed in a Backwater Dungeon: Volume 4 (Light Novel) by Meikyou Shisui

After crossing Sionne off his revenge list and sealing the tremor-causing interdimensional portal for good, Light returns to the Abyss, where his debrief is interrupted by news from Mei. She tells Light they’ve found his younger sister, Yume, who went missing after someone—or something—destroyed their hometown. It turns out she is still in the Human Kingdom after being rescued by Princess Lilith, who kept her on as a maid. While Light makes plans to bring Yume to the Abyss, he reminisces about his life since they last saw each other. He recounts the days after he was betrayed, going from merely surviving the Abyss with Mei to pushing past his limits, summoning more overpowered allies, and eventually transforming the world’s more notorious dungeon into his very own hidden kingdom. When Light finally meets Princess Lilith himself, they find they share a passion for improving humanity’s lot—but a dark secret plagues her kingdom.