I am trying to remember a xianxia harem novel. I do not remember the many of the female leads but I distinctly remember a character. In the second arc.
I think she was an assassin whom the mc befriends in the first arc due to the mc knowing the whereabouts of her brother and his death. After the arc finished, the mc set out with her and i think another girl(?) to a secret realm to find her brother's corpse and proof of his death. To do so, they had to willingly entered an formation where they teach those stuck in the formation about the "Dao". Later on, the mc explained that it was simply a trap and those stuck inside were forced to go through repetitive days until they fell into madness. They can also see illusions of others who were trapped there and how they descended into insanity and died. There they saw the assassin's brother and observed him to his deathbed. After he died, the mc broke the illusion.