The Twilight Obelisk (Mirror World Book #4): LitRPG series

Olgerd and his tribal NPC gang have finally discovered the Forbidden City. Problem is, they haven't found what they were looking for. The ancient maps are hopelessly out of date and the Ennan capital city is lying in ruins.In the meantime, their enemies are closing in. Noctean hordes are arriving from the Silver Mountain Valley; the Dark army has already crossed the Back Stream, and the forces of Light have already entered the Icy Woods. The magic protection sphere which Olgerd received from the city's invisible guardians is about to expire... The only way he can save the lives of his trusty NPC friends is by activating the Twilight Obelisk.